Life of Sir Isaac Newton 1840

8/2/2019 Life of Sir Isaac Newton 1840

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2IR.G.KIIELLER I


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THELIFEOFSIR ISAAC NEWTONBY

DAVID BREWSTER, LL.D. F.R.S.Ergo vivida vis anim? pervicit, et extraProcessit longe flamnwntia moenia mundi ;Atque omoe immensum peragravit meote animoque.

Lttcret. lib. i. 1. 731

The Birthplace of Ncwtoa.NEW-YORK:

PUBLISHED BY HARPER & BROTHERS,NO. 82 C L I F F-S T R E K T.

1840.


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TOTHE RIGHT HONOURABLE

LORD BRAYBROOKE.THE kindness with which your lordship intrusted

to me some very valuable materials for the compo-sition of this volume has induced me to embrace thepresent opportunity of publicly acknowledging it.But even if this personal obligation had been lesspowerful, those literary attainments and that en-lightened benevolence which reflect upon rank itshighest lustre would have justified me in seekingfor it the patronage of a name which they have sojustly honoured* DAVID BREWSTER.

Allerly, June 1st, 1831.


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PREFACE.As this is the only Life of Sir Isaac Newton on

any considerable scale that has yet appeared, Ihave experienced great difficulty in preparing it forthe public. The materials collected by precedingbiographers were extremely scanty ; the particularsof his early life, and even the historical details ofhis discoveries, have been less perfectly preservedthan those of his illustrious predecessors ; and it isnot creditable to his disciples that they have alloweda whole century to elapse without any suitablerecord of the life and labours of a master whounited every claim to their affection and gratitude.

In drawing up this volume, I have obtained muchassistance from the account of Sir Isaac Newton inthe Biographia Britannica ; from the letters to Ol-denburg, arid other papers in Bishop Horsley'sedition of his works ; from Tumor's Collections forthe History of the Town and Soke of Grantham ;from M. Biot's excellent Life of Newton in theBiographic Universelle ; and from Lord King's Lifeand Correspondence of Locke.

Although these works contain much importantinformation respecting the Life of Newton, yet Ihave been so fortunate as to obtain many new ma-terials of considerable value.


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10 PREFACE.To the kindness of Lord Braybrooke I have beenindebted for the interesting correspondence of New-

ton, Mr. Pepys, and Mr. Millington, which is nowpublished for the first time, and which throws muchlight upon an event in the life of our author thathas recently acquired an unexpected and a painfulimportance. These letters, when combined withthose which passed between Newton and Locke,and with a curious extract from the manuscript diaryof Mr. Abraham Pryme, kindly furnished to me byhis collateral descendant Professor Pryme of Cam-bridge, fill up a blank in his history, and have ena-bled me to delineate in its true character that tem-porary indisposition which, from the view that hasbeen taken of it by foreign philosophers, has beenthe occasion of such deep distress to the friends ofscience and religion.To Professor Whewell, of Cambridge, I owe verygreat obligations for much valuable information.Professor Rigaud, of Oxford, to whose kindness Ihave on many other occasions been indebted, supplied me with several important facts, and with extracts from the diary of Hearne in the BodleianLibrary, and from the original correspondence be-tween Newton and Flamstead, which the presidentof Corpus Christi College had for this purpose com-mitted to his care ; and Dr. J. C. Gregory, of Edin-burgh, the descendant of the illustrious inventor ofthe reflecting telescope, allowed me to use his un-published account of an autograph manuscript ofSir Isaac Newton, which was found among the pa-pers of David Gregory, Savilian Professor of As-tronomy at Oxford, and which throws some light onthe history of the Principia.


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PREFACE. 11I have been indebted to many other friends for

the communication of books and facts, but espe-cially to Sir William Hamilton, Bart., whose libe-rality in promoting literary inquiry is not limited tothe circle of his friends.

D.B.Alhrly, June 1st, 1831.


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CONTENTS.CHAPTER I. PagThe Pre-eminence of Sir Isaac Newton's Reputation The Interestattached to the Study of his Life and Writings His Birth and

Parentage His e^rly Education Is sent to Grantham SchoolHis early Attachment to Mechanical Pursuits His WindmillHis Water-clock His Self-moving Cart His Sun-dials HisPreparation for the University H

CHAPTER II.Newton enters Trinity College, Cambridge Origin of his Propen-

sity for Mathematics He studies the Geometry of Descartes unas-sisted Purchases a Prism Revises Dr. Barrow's Optical Lec-tures Dr. Barrow's Opinion respecting Colours Takes his De-grees Is appointed a Fellow of Trinity College Succeeds Dr.Barrow in the Lucasian Chair of Mathematics 26

CHAPTER III.Newton occupied in grinding Hyperbolical Lenses His first Ex-periments with the Prism made in 1666 He discovers the Com-position of White Light, and the different Refrangibility of theRays which compose it Abandons his Attempts to improve Re-fracting Telescopes, and resolves to attempt the Construction ofReflecting ones He quits Cambridge on account of the PlagueConstructs two Reflecting Telescopes in 1668, the first ever exe-cutedOne of them examined by the Royal Society, and shownto the King He constructs a Telescope with Glass Specula Re-eent History ofthe Reflecting Telescope Mr. Airy's Glass SpeculaHadley's Reflecting Telascopes Short's Berschel's -Ram-age*s Lord Oxmantown's 30

CHAPTER IV.He delivers a Course of Optical Lectures at Cambridge Is electedFellow of the Royal Society He communicates to them his Dis-coveries on the different Refrangibility and Nature of Light-Popular Account of them They involve him in various Contro-versiesHis Dispute with Pardies Linus Lucas Dr. Hooke. and Mr. Huygens The Influence of these Disputes on the mindof Newton 47

CHAPTER V.Mistake of Newton in supposing that the Improvement of Befraot-B


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14 CONTENTS.Paging Telescopes was hopeless Mr. Hall invents the Achromatic

Telescope Principles of the Achromatic Telescope explained Itis reinvented by Dollond, and improved by future Artists Dr.Blair's Aplanatic Telescope Mistakes in Newton's Analysis ofthe Spectrum Modern Discoveries respecting the Structure ofthe Spectrum 63

CHAPTER VI.Colours of thin Plates first studied by Boyle and Hooke Newtondetermines the Law of their Production His Theory of Fits ofeasy Reflection and Transmission Colours of thick Plates 75

CHAPTER VII.Newton's Theory of the Colours of Natural Bodies explained Ob-

jections to it stated New Classification of Colours Outline of anew Theory proposed 89CHAPTER VIII.

Newton's Discoveries respecting the Inflection or Diffraction ofLight Previous Discoveries ofGrimaldi and Dr. Hooke Laboursof succeeding Philosophers Law of Interference of Dr. YoungFresnel's Discoveries New Theory of Inflection on the Hypothe-sis of the Materiality of Light 99

CHAPTER IX.Miscellaneous Optical Researches of Newton His Experiments onRefraction His Conjecture respecting the Inflammability of theDiamond His Law of Double Refraction His Observations on

the Polarization of Light Newton's Theory of Light His " Op-tics" 106

CHAPTER X.Astronomical Discoveries of Newton Necessity of combined Exer

tion to the completion of great Discoveries Sketch of the Historyof Astronomy prexrfous to the time of Newton Copernicus, 1473-1543 TychoBrahe, 1546-1601 Kepler, 1571-1631 Galileo, 1564-1642 110

CHAPTER XI.The first Idea of Gravity occurs to Newton in 1666 His first Specu-lations upon it Interrupted by his Optical Experiments Heresumes the Subject in consequence of a Discussion with DoctorHooke He discovers t'je true Law of Gravity and the Cause ofthe Planetary Motions Dr. Halley urges him to publish his Prin-

cipia His Principles of Natural Philosophy Proceedings of theRoyal Society on this Subject The Principia appears in 1687General Account of it, and of the Discoveries it contains Theymeet with great Opposition, owing to the Prevalence of the Carte-sian System Account of the Reception and Progress of the New-tonian Philosophy in Foreign Countries Account of its Progressand Establishment in England 140


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CONTENTS. 15CHAPTER XIL Paga

Doctrine of Infinite Quantities Labours ofPappus Kepler Cava-leri Roberval Fermat Wallis Newton discovers the Bino-mial Theorem and the Doctrine of Fluxions in 16C6 His Manu-script Work containing this Doctrine communicated to his FriendsHis Treatise on Fluxions His Mathematical Tracts His Uni-versal Arithmetic His Methodus Differentials His GeornetriaAnalytica His Solution of the Problems proposed by Bernouilliand Leibnitz Account of the celebrated Dispute respecting theInvention of Fluxions Commercium Epistolicurn Report of theRoyal Society General View of the Controversy 168

CHAPTER Xin.James IT. attacks the Privileges of the University of CambridgeNewton chosen one of the Dtlegates to resist this EncroachmentHe is elected a Member of the Convention Parliament Burningof his Manuscript His supposed Derangement of Mind Viewtaken of this by foreign Philosophers His Correspondence withMr. Pepys and Mr. Locke at the time of his Illness Mr. Milling-ton's Letter to Mr. Pepys on the subject of Newton's IllnessRefutation of the Statement that he laboured under Mental De-rangement 200

CHAPTER XIV.No Mark of National Gratitude conferred upon Newton Friendshipbetween him and Charles Montague, afterward Earl of HalifaxMr. Montague appointed Chancellor of the Exchequer in 1604He resolves upon a Recoinage Nominates Mr. Newton Warden

of the Mint in 1695 Mr. Newton appointed Master of the Mint in1699 Notice of the Earl of Halifax Mr. Newton elected Asso-ciate of the Academy of Sciences in 1699 Member for Cambridgein 1701 and President cf the Royal Society in 1703 Queen Anneconfers upon him the Honour of Knighthood in 1705 SecondEdition of the Principia, edited by Cotes His Conduct respectvjgMr. Ditton's Method of finding the Longitude 223

CHAPTER XV.Respect in which Newton was held at the Court of George I. The

Princess of Wales delighted with his Conversation Leibnitz en-deavours to prejudice the Princess against Sir Isaac and Locke 'Controversy occasioned by his Conduct The Princess obtains aManuscript Abstract of his System of Chronology The AbbeConti is, at her request, allowed to take a Copy of it on the prom-ise of Secrecy He prints it surreptitiously in French, accompa-nied with a Refutation by M. Freret Sir Isaac's Defence of hisSystem Father Souciet attacks it, and is answered by Dr. HalleySir Isaac's larger Work on Chronology published afler hisDeathOpinions respecting it Sir Isaac's Paper on the Form ofthe most ancient Year 234

CHAPTER XVI.Theological Studies of Sir Isaac Their Importance to Christianity-Motives to which they have been ascribed Opinions of Biot


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16 CONTENTS.Faffand La Place considered His Theological Researches begun be-

fore his supposed Mental Illness The Date of these Works fixedLetters to Locke Account of his Observations on ProphecyHis Lexicon Propheticum His Four Letters to Dr. Bentley Origin of Newton's Theological Studies Analogy between the Bookof Nature and that of Revelation 242

CHAPTER XVH.The Minor Discoveries and Inventions of Newton His Researcheson Heat On Fire and Flame On Elective Attraction On the

Structure of Bodies His supposed Attachment to Alchymy HisHypothesis respecting Ether as the Cause of Light and GravityOn the Excitation of Electricity in Glass His Reflecting Sex-tant invented before 1700 His Reflecting Microscope His Pris-matic Reflector as a Substitute for the small Speculum of Reflect-ing Telescopes His Method of varying the Magnifying Power ofNewtonian Telescopes His Experiments on Impressions on theRetina 265

CHAPTER XVIII.His Acquaintance with Dr. Pemberton Who edits the Third Edi-tion of the Principia His first Attack of ill Health His RecoveryHe is taken ill in consequence of attending the Royal SocietyHis Death on the 20th March, 1727 His Body lies in state HisFuneral He is buried in WestminsterAbbey His Monument de-scribed His Epitaph A Medal struck in honour of him Roubil-

iac's full-length Statue of him erected in Cambridge Divisionof his Property His Successors 284CHAPTER XIX.

Permanence of Newton's Reputation Character of his GeniusHis Method of Investigation similar to that used by GalileoError in ascribing his Discoveries to the Use of the Methodsrecommended by Lord Bacon The Pretensions of the BaconianPhilosophy examined Sir Isaac Newton's Social Character Hisgreat Modesty The Simplicity of his Character His Religiousand Moral Character His Hospitality and Mode of Life HisGenerosity and Charity His Absence His Personal Appear-anceStatues and Pictures of him Memorials and Recollectionsof him 292

APPENDIX, No. I. Observations on the Family of Sir Isaac New-ton 307APPENDIX, No. n. Letter from Sir Isaac Newton to Francis Aston,

Esq., a young Friend who was on the eve of setting out on hisTravels 318APPENDIX, No. HI. " A Remarkable and Curious Conversation be-tween Sir Isaac Newton and Mr. Conduit." 320


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LIFEoff

SIR ISAAC NEWTON.CHAPTER I.

The Pre-eminence of Sir Isaac Newton's Reputation The Interestattached to the Study of his Life and Writings His Birth andParentage His early Education Is sent to Grantham School Hisearly Attachment to Mechanical Pursuits His WindmillHisWaterclock His Selfmoving Cart His Sundials His Preparationfor the University.THE name of Sir Isaac Newton has by generalconsent been placed at the head of those great men

who have been the ornaments of their species.However imposing be the attributes with whichtime has invested the sages and the heroes of anti-quity, the brightness of their fame has been eclipsed,by the splendour of his reputation ; and neither thepartiality of rival nations, nor the vanity of a presump-tuous age, has ventured to dispute the ascendencyof his genius. The philosopher,* indeed, to whomposterity will probably assign the place next to New-ton, has characterized the Principia as pre-eminentabove all the productions of human intellect, andhas thus divested of extravagance the contemporaryencomium upon its author,

Nee fas est propius mortal! attingere Divos.HALLET.So near the gods man cannot nearer go.I .

* The Marquis La Place. See Systtme du Monde, p. 336.B2


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18 SIR ISAAC NEWTONThe biography of an individual so highly renownedcannot fail to excite a general interest. Though

his course may have lain in the vale of private life,and may have been unmarked with those dramaticevents which throw a lustre even round perishablenames, yet the inquiring spirit will explore the his-tory of a mind so richly endowed, will study itsintellectual and moral phases, and will seek theshelter of its authority on those great questionswhich reason has abandoned to faith and hope.

If the conduct and opinions of men of ordinarytalent are recorded for our instruction, how inter-esting must it be to follow the most exalted geniusthrough the incidents of common life ; to mark thesteps by which he attained his lofty pre-eminence ;to see how he performs the functions of the socialand the domestic compact; how he exercises hislofty powers of invention and discovery; how hecomports himself in the arena of intellectual strife ;and in what sentiments, and with what aspirationshe quits the world which he has adorned.In almost all these bearings, the life and writingsof Sir Isaac Newton abound with the richest counsel.Here the philosopher will learn the art by whichalone he can acquire an immortal name. The mor-alist will trace the lineaments of a character ad-justed to all the symmetry of which our imperfectnature is susceptible; and the Christian will con-template with delight the high-priest of sciencequitting the study of the material universe, thescene of his intellectual triumphs, to investigatewith humility and patience the mysteries of his faith.

Sir Isaac Newton was born at Woolsthorpe, ahamlet in the parish of Colsterworth, in Lincoln-shire, about six miles south of Grantham, on the25th December, O. S., 1642, exactly one year afterGalileo died, and was baptized at Colsterworth onthe 1st January, 1643-3. His father, Mr. Isaac New-


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BIRTH. 19ton, died at the early age of thirty-six, a little morethan a year after the death of his father RobertNewton, and only a few months after his marriageto Harriet Ayscough, daughter of James Ayscoughof Market Overton in Rutlandshire. This lady wasaccordingly left in a state of pregnancy, and appearsto have given a premature birth to her only andposthumous child. The helpless infant thus usheredinto the world WPS of such an extremely diminutivesize,* and seemed of so perishable a frame, thattwo women who were sent to Lady Pakenham's atNorth Witham, to bring some medicine to strengthenhim, did not expect to find him alive on their return.Providence, however, had otherwise decreed; andthat frail tenement which seemed scarcely able toimprison its immortal mind was destined to enjoya vigorous maturity, and to survive even the averageterm of human existence. The estate of Wools-thorpe, in the manor-house of which this remarka-ble birth took place, had been more than a hundredyears in the possession of the family, who cameoriginally from Newton in Lancashire, but who had,previous to the purchase of Woolsthorpe, settled atWestby, in the county of Lincoln. The manor-house, of which we have given an engraving, issituated in a beautiful little valley, remarkable forits copious wells of pure spring water, on the westside of the river Witham, which has its origin in theneighbourhood, and commands an agreeable pros-pect to the east towards Colsterworth. The manorof Woolsthorpe was worth only 30Z. per annum;but Mrs. Newton possessed another small estate atSewstern,f which raised the annual value of theirproperty to about 80Z. ; and it is probable that thecultivation of the little farm on which she resided

* Sir Isaac Newton told Mr. Conduit, that he had often heard hismother say that when he was born he was so little that they mighthave put him into a quart mug.t In Leicestershire, and about three miles south-east of Woolsthorpe.


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20 SIR ISAAC NEWTON.somewhat enlarged the limited income upon whichshe had to support herself, and educate her child.For three years Mrs. Newton continued to watchover her tender charge with parental anxiety ; but

in consequence of her marriage to the ReverendBarnabas Smith, rector of North Witham, about amile south of Woolsthorpe, she left him under thecare of her own mother. At the usual age he wassent to two day-schools at Skillington and Stoke,where he acquired the education which such semi-naries afforded; but when he reached his twelfthyear he went to the public school at Grantham,taught by Mr. Stokes, and was boarded at the houseof Mr. Clark, an apothecary in that town. Accord-ing to information which Sir Isaac himself gave toMr. Conduit, he seems to have been very inattentiveto his studies, and very low in the school. Theboy, however, who was above him, having one daygiven him a severe kick upon his stomach, fromwhich he suffered great pain, Isaac laboured inces-santly till he got above him in the school, and fromthat time he continued to rise till he was the headboy. From the habits of application which thisincident had led him to form, the peculiar characterof his mind was speedily displayed. During thehours of play, when the other boys were occupiedwith their amusements, his mind was engrossed withmechanical contrivances, either in imitation of some-thing which he had seen, or in execution of someoriginal conception of his own. For this purposehe provided himself with little saws, hatchets, ham-mers, and all sorts .of tools, which he acquired theart -of using with singular dexterity. The principalpieces of mechanism which he thus constructedwere a windmill, a waterclock, and a carriage put inmotion by the person who sat in it. When a wind-mill was erecting near Grantham on the road toGunnerby, Isaac frequently attended the operationsof the workmen, and acquired such a thorough


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MECHANICAL PURSUITS. 21knowledge of the machinery that he completed aworking model of it, which excited universal admi-ration. This model was frequently placed on the topof the house in which he lodged at Grantham, andwas put in motion by the action of the wind uponits sails. Not content with this exact imitation ofthe original machine, he conceived the idea of driv-ing it by animal power, and for this purpose he en-closed in it a mouse which he called the miller, andwhich, by acting upon a sort of treadwheel, gavemotion to the machine. According to some ac-counts, the mouse was made to advance by pullinga string attached to its tail, while others allege thatthe power of the little agent was called forth by itsunavailing attempts to reach a portion of corn placedabove the wheel.His waterclock was formed out of a box whichhe had solicited from Mrs. Clark's brother. It wasabout four feet high, and of a proportional breadth,somewhat like a common houseclock. The indexof the dialplate was turned by a piece of wood, whicheither fell or rose by the action of dropping water.As it stood in his own bedroom he supplied it everymorning with the requisite quantity of water, and itwas used as a clock by Mr. Clark's family, and re-mained in the house long after its inventor hadquitted Grantham.* His mechanical carriage was avehicle with four wheels, which was put in motionwith a handle wrought by the person who sat in it,but, like Merlin's chair, it seems to have been usedonly on the smooth surface of a floor, and not fittedto overcome the inequalities of a road. Although

etween the loose specula of Gregory and the fineGregorian telescopes of Hadley, Short, and Veitch,-between the humble six-inch tube of Newton andthe gigantic instruments of Herschel and Ramage.


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40 SIR ISAAC NEWTON.The success of this first experiment inspired New-ton with fresh zeal, and though his mind was now

occupied with his optical discoveries, with the ele-ments of his method of fluxions, and with the ex-panding germ of his theory of universal gravitation,yet with all the ardour of youth he applied himselfto the laborious operation of executing another re-flecting telescope with his own hands. This instru-ment, which was better than the first, though it layby him several years, excited some interest at Cam-bridge ; and Sir Isaac himself informs us, that oneof the fellows of Trinity College had completed atelescope of the same kind, which he considered assomewhat superior to his own. The existence ofthese telescopes having become known to the RoyalSociety, Newton was requested to send his instru-ment for examination to that learned body. He ac-cordingly transmitted it to Mr. Oldenburg in Decem-ber, 1671, and from this epoch his name began toacquire that celebrity by which it has been so pecu-liarly distinguished.On the llth of January, 1672, it was announcedto the Royal Society that his reflecting telescopehad been shown to the king, and had been examined"by the president, Sir Robert Moray, Sir Paul Neale,Sir Christopher Wren, and Mr, Hook. These gen-tlemen entertained so high an opinion of it, that, inqjder to secure the honour of the contrivance to itsauthor, they advised the inventor to send a drawingand description of it to Mr. Huygens at Paris. Mr.Oldenburg accordingly drew up a description of itin Latin, which, after being corrected by Mr. New-ton, was transmitted to that eminent philosopher.This telescope, of which the annexed is an accuratedrawing, is carefully preserved in the library of theRoyal Society of London, with the following in-scription :

" Invented by Sir Isaac Newton and made with hisown hands 1671."


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42 SIR ISAAC NEWTOX.It does not appear that Newton executed any other

reflecting telescopes than the twowe have mentioned.He informs us that he repolished and greatly im-proved a fourteen-feet object-glass, executed by aLondon artist, and having proposed in 1678 to sub-stitute glass reflectors in place of metallic specula,he tried to make a reflecting telescope on thisprinciple four feet long, and with a magnifying powerof 150. The glass was wrought by a London artist,and though it seemed well finished, yet, when it wasquicksilvered on its convex side, it exhibited all overthe glass innumerable inequalities, which gave anindistinctness to every object. He expresses, how-ever, his conviction that nothing but good work-manship is wanting to perfect these telescopes, andhe recommends their consideration " to the curiousin figuring glasses."For a period of fifty years this recommendationexcited no notice. At last Mr. James Short ofEdinburgh, an artist of consummate skill, executedabout the year 1730 no fewer than six reflectingtelescopes with glass specula, three of fifteen inches,and three of nine inches in focal length. He foundit extremely troublesome to give them a true figurewith parallel surfaces ; and several of them whenfinished turned out useless, in consequence of theveins which then appeared in the glass. Althoughthese instruments performed remarkably well, yetthe light was fainter than he expected, and from thiscause, combined with the difficulty of finishing them,he afterward devoted his labours solely to thosewith metallic specula.At a later period, in 1822, Mr. G. B. Airy ofTrinity College, and one of the distinguished suc-cessors of Newton in the Lucasian chair, resumedthe consideration of glass specula, and demonstratedthat the aberration both of figure and of colourmight be corrected in these instruments. Upon thisingenious principle Mr. Airy executed more than


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REFLECTING TELESCOPES. 43one telescope, but though the result of the experi-ment was such as to excite hopes of ultimate suc-cess, yet the construction of such instruments isstill a desideratum in practical science.Such were the attempts which Sir Isaac Newtonmade to construct reflecting telescopes ; but notwith-standing the success of his labours, neither the phi-losopher nor the practical optician seems to havehad courage to pursue them. A London artist, in-deed, undertook to imitate these instruments ; butSir Isaac informs us, that " he fell much short ofwhat he had attained, as he afterward understoodby discoursing with the under workmen he had em-ployed." After a long period of fifty years, JohnHadley, Esq. of Essex, a Fellow of the Royal So-ciety, began in 1719 or 1720 to execute a reflectingtelescope. His scientific knowledge and his manualdexterity fitted him admirably for such a task, and,probably after many failures, he constructed twolarge telescopes about five feet three inches long,one of which, with a speculum six inches in diameter,was presented to the Royal Society in 1723. Thecelebrated Dr. Bradley and the Rev. Mr. Poundcompared it with the great Huygenian refractor 123feet long. It bore as high a magnifying power asthe Huygenian telescope : it showed objects equallydistinct, though not altogether so clear and bright,and it exhibited every celestial object that had beendiscovered by Huygens, the five satellites of Sat-urn, the shadow of Jupiter's satellites on his disk,the black list in Saturn's ring, and the edge of hisshadow cast on the ring. Encouraged and instructedby Mr. Hadley, Dr. Bradley began the constructionof reflecting telescopes, and succeeded so well thathe woAid have completed one of them, had he notbeen obliged to change his residence. Some timeafterward he and the Honourable Samuel Molyneuxundertook the task together at Kew, and attemptedto execute specula about twenty-six inches in focal


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44 SIR ISAAC NEWTON.length ; but notwithstanding Dr. Bradley's formerexperience, and Mr. Hadley's frequent instructions,it was a long time before they succeeded. The firstgood instrument which they finished was in May,1724. It was twenty-six inches in focal length ; butthey afterward completed a very large one of eightfeet, the largest that had ever been made. The firstof these instruments was afterward elegantly fittedup by Mr. Molyneux, and presented to Ins majestyJohn V. King of Portugal.The great object of these two able astronomerswas to reduce the method of making specula tosuch a degree of certainty that they could be manu-factured for public sale. Mr. Hauksbee had indeedmade a good one about three and a half feet long,and had proceeded to the execution of two others,one of six feet, and another of twelve feet in focallength ; but Mr. Scarlet and Mr. Hearne, having re-ceived all the information which Mr. Molyneux hadacquired, constructed them for public sale ; and thereflecting telescope has ever since been an article oftrade with every regular optician.As Sir Isaac Newton was at this time Presidentof the Royal Society, he had the high satisfactionof seeing his own invention become an instrumentof public use, and of great advantage to science,and he no doubt felt the full influence of this triumphof his skill. Still, however, the reflecting telescopehad not achieved any new discovery in the heavens.The latest accession to astronomy had been madeDy the ordinary refractors of Huygens, labouringunder all the imperfections of coloured light ; andthis long pause in astronomical discovery seemedto indicate that man had carried to its farthest limitshis power of penetrating into the depths of the uni-verse. This, however, was only one of those sta-tionary positions from which human genius takes anew and a loftier elevation. While the English op-ticians were thus practising the recent art of grinding


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DR. HEUSCHEL'S TELESCOPES. 45specula, Mr. James Short of Edinburgh was devot-ing to the subject all the energies of his youthfulmind. In 1732, and in the 22d year of his age, hebegan his labours, and he carried to such high per-fection the art of grinding and polishing specula, andof giving them the true parabolic figure, that, with.a telescope fifteen inches in focal length, he read inthe Philosophical Transactions at the distance of500 feet, and frequently saw the five satellites ofSaturn together, a power which was beyond thereach even of Hadley's six-feet instrument. Thecelebrated Maclaurin compared the telescopes ofShort with those made b}^ the best London artists,and so great was their superiority, that his smalltelescopes were invariably superior to larger onesfrom London. In 1742, after he had settled as anoptician in the metropolis, he executed for LordThomas Spencer a reflecting telescope, twelve feetin focal length, for 630/. ; in 1752 he completed one forthe King of Spain, at the expense of 1200/. ; and ashort time before his death, which took place in1768, he finished the specula of the large telescopewhich was mounted equatorially for the observatoryof Edinburgh by his brother Thomas Short, who wasoffered twelve hundred guineas for it by the King ofDenmark.Although the superiority of these instruments,which were all of the Gregorian form, demonstratedthe value of the reflecting telescope, yet no skilfulhand had yet directed it to the heavens ; and it wasreserved for Dr. Herschel to employ it as an instru-ment of discovery, to exhibit to the eye of man newworlds and new systems, and to bring within the

grasp of his reason those remote regions of spaceto which his imagination even had scarcely venturedto extend its power. So early as 1774 he completeda five-feet Newtonian reflector, and he afterwardexecuted no fewer than two hundred 7 feet, one hun-dred and fifty 10 feet, and eighty 20 feet specula. In


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"46 SIR ISAAC NEWTON.1781 he began a reflector thirty feet long, and havinga speculum thirty-six inches in diameter ; and underthe munificent patronage of George III. he com-pleted, in 1789, his gigantic instrument forty feetlong, with a speculum forty-nine and a half inchesin diameter. The genius and perseverance whichcreated instruments of such traiiscendant magnitudewere not likely to terminate with their construction.In the examination of the starry heavens, the ulti-mate object of his labours. Dr. Herschel exhibitedthe same exalted qualifications, and in a few yearshe rose from the level of humble life to the enjoy-ment of a name more glorious than that of the sagesand warriors of ancient times, and as immortal asthe objects with which it will be for ever associated.Nor was it in the ardour of the spring of life thatthese triumphs of reason were achieved. Dr. Her-schel had reached the middle of his course beforehis career of discovery began, and it was in the au-tumn and winter of his days that he reaped the fullharvest of his glory. The discovery of a new planetat the verge of the solar system was the first trophyof his skill, and new double and multiple stars, andnew nebulae, and groups of celestial bodies wereadded in thousands to the system of the universe.The spring-tide of knowledge which was thus let inupon the human mind continued for a while to spreadits waves over Europe ; but when it sank to its ebb inEngland, there was no other bark left upon the strandbut that of the Deucalion of Science, whose homehad been so long upon its waters.During the life of Dr. Herschel, and during the

reign, and within the dominions of his royal patron,four new planets were added to the solar system,but they were detected by telescopes of ordinarypower ; and we venture to state, that since the reignof George III no attempt has been made to keepup the continuity of Dr. Herschel's discoveries.Mr. Herschel, his distinguished son, has indeed


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ELECTED INTO THE ROYAL SOCIETY. 47c **pleted more than one telescope of considerablesize ; Mr. Ramage, of Aberdeen, has executed re-flectors rivalling almost those of Slough ; and LordOxmantown, an Irish nobleman of high promise, isnow engaged on an instrument of great size. Butwhat avail the enthusiasm and the efforts of indi-vidual minds in the intellectual rivalry of nations ?When the proud science of England pines in obscu-rity, blighted by the absence of the royal favour, andof the nation's sympathy; when its chivalry fallunwept and unhonoured ; how can it sustain theconflict against the honoured and marshalled geniusof foreign lands ?

CHAPTER IV.He delivers a Course of. Optical Lectures at Cambridge Is elected Fellowof the Royal Society He communicates to them his Discoveries onthe different Refrangibility and Nature of LightPopular Accountof them They involve him in various Controversies His Disputewith Pardies Linus Lucas Dr. Hooke and Mr. HuygensTh*Influence of these Disputes on the Mind of Newton.ALTHOUGH Newton delivered a course of lectureson optics in the University of Cambridge in the

years 1669, 1670, and 1671, containing his principaldiscoveries relative to the different refrangibility oflight, yet it is a singular circumstance, that thesediscoveries should not have become public throughthe conversation or correspondence of his pupils.The Royal Society had acquired no knowledge ofthem till the beginning of 1672, and his reputation

; in that body was founded chiefly on his reflecting'telescope. On the 23d December, 1671, the cele-brated Dr. Seth Ward, Lord Bishop of Sarum, whowas the author of several able works on astronomy,and had filled the astronomical chair at Oxford,proposed Mr. Newton as a Fellow of the Royal


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48 SIR ISAAC NEWTON.Society. The satisfaction which he derived from thiscircumstance appears to have been considerable ;and in a letter to Mr. Oldenburg, of the 6th January,he says, " I am very sensible of the honour doneme by the Bishop of Sarum in proposing me a can-didate ; and which, I hope, will be further conferredupon me by my election into the Society ; and ifso, I shall endeavour to testify my gratitude, bycommunicating what my poor and solitary endeav-ours can effect towards the promoting your philo-sophical designs." His election accordingly tookplace on the llth January, the same day on whichthe Society agreed to transmit a description of histelescope to Mr. Huygens at Paris. The notice ofhis election, and the thanks of the Society for thecommunication of his telescope, were conveyed inthe same letter, with an assurance that the Society" would take care that all right should be done himin the matter of this invention." In his next letterto Oldenburg, written on the 18th January, 1671-2,he announces his optical discoveries in the followingremarkable manner : " 1 desire that in your nextletter you would inform me for what time the So-ciety continue their weekly meetings ; because ifthey continue them for any time, I am purposingthem, to be considered of and examined, an accountof a philosophical discovery which induced me to themaking of the said telescope ; and I doubt not butwill prove much more grateful than the communica-tion of that instrument ; being in my judgment theoddest, if not the most considerable detection whichhath hitherto been made in the operations of nature."This " considerable detection" was the discoveryof the different refrangibility of the rays of lightwhich we ^ave already explained, and which ledto the construction of his reflecting telescope. Itwas communicated to the Royal Society in a letterto Mr. Oldenburg, dated February 6th, and excitedgreat interest among its members. The "solemn


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COMMUNICATES HIS DISCOVERIES. 49thanks" of the meeting were ordered to be trans-mitted to its author for his "very ingenious dis-course." A desire was expressed to have it imme-diately printed, both for the purpose of having itwell considered by philosophers, and for " securingthe considerable notices thereof to the author againstthe arrogations of others ;" and Dr. Seth Ward,Bishop of Salisbury, Mr. Boyle, and Dr. Hookewere desired to peruse and consider it, and to bringin a report upon it to the Society.The kindness of this distinguished body, and theanxiety which they had already evinced for hisreputation, excited on the part of Newton a corres-ponding feeling, and he gladly accepted of their pro-posal to publish his discourse in the monthly num-bers in which the Transactions were then given tothe world. " It was an esteem," says he,* " of theRoyal Society for most candid and able judges inphilosophical matters, encouraged me to presentthem with that discourse of light and colours, whichsince they have so favourably accepted of, I do ear-nestly

desire you to return them my cordial thanks.I before thought it a great favour to be made amember of that honourable body; but I am nowmore sensible of .the advantages; for believe me,sir, I do not only esteem it a duty to concur withyou in the promotion of real knowledge ; but a greatprivilege, that, instead of exposing discourses to aprejudiced and common multitude, (by which meansmany truths have been baffled and lost), I may withfreedom apply myself to so jpdicious and impartialan assembly. A s to the printing of that letter, I amsatisfied in theii judgment, or else I should havethought it too straight and narrow for public view.I designed it only to those that know how to im-prove upon hints of things ; and, therefore, to sparetediousness, omitted many such remarks and ex*

* Letter to Oldenburg, February 10, 1671.E


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50 SIR ISAAC NEWTON.periments as might be collected by considering theassigned laws of refractions ; some of which I be-lieve, with the generality of men, would yet bealmost as taking as any I described. But yet, sincethe Royal Society have thought it fit to appear pub-licly, 1 leave it to their pleasure : and perhaps tosupply the aforesaid defects, I may send you somemore of the experiments to second it (if it be sothought fit), in the ensuing Transactions."Foliowing the orderwhich Newton himselfadopted,we have, in the preceding chapter, given an accountof the leading doctrine of the different refrangibilityof light, and of the attempts to improve the reflect-ing telescope which that discovery suggested. Weshall now, therefore, endeavour to make the readeracquainted with the other discoveries respectingcolours which he at this time communicated to theRoyal Society.Having determined, by experimentsalready described, that a beam ofwhite light, as emitted from the sun,consisted of seven different colours,which possess different degrees ofrefrangibility, he measured the re-lative extent of the coloured sp^es,and found them to have the propor-tions shown in Jig. 4, which rep-resents the prismatic spectrum, and

, u hich is nothing more than an elon-gated image of the sun produced bythe rays being separated in differentdegrees from their original direction,the red being refracted least, and theviolet most powerfully.

If we consider light as consistingof minute particles of matter, we mayform some notion of its decomposi-tion by the prism from the followingpopular illustration. If we take steel

Fig. 4.

OrangeYellow

Green

Blue

Indigo

Violet


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DECOMPOSITION OF lIGHT. 51filings of seven different degrees of fineness andmix them together, there are two ways in which we

i may conceive the mass to be decomposed, or, whatI is the same thing, all the seven different kinds of| filings separated from each other. By means of[seven sieves of different degrees of fineness, and soImade that the finest will just transmit the finestpowder and detain all the rest, while the next infineness transmits the two finest powders and detainsall the rest, and so on, it is obvious that all the pow-ders may be completely separated from each other.If we again mix all the steel filings, and laying themupon a table, hold high above them a flat bar magnet,so that none of the filings are attracted, then if webring the magnet nearer and nearer, we shall cometo a point where the finest filings are drawn up to it.These being removed, and the magnet broughtnearer still, the next finest powders will be attracted,and so on till we have thus drawn out of the massall the powders in a separate state. We may con-ceive the bar magnet to be inclined to the surfaceof the steel filings, and so moved over the mass,that at the end nearest to them the heaviest orcoarsest will be attracted, and all the remotest andthe finest or ligfflrer filings, while the rest are at-tracted to intermediate points, so that the sevendifferent filings are not only separated, but are foundadhering in separate patches to the surface of theflat magnet. The first of these methods, with thesieves, may represent the process of decomposinglight, by which certain rays of white light are ab-sorbed, or stifled, or stopped in passing throughbodies, while certain other rays are transmitted.The second method may represent the process ofdecomposing light by refraction, or by the attractionof certain rays farther from their original directionthan other rays, and the different patches of filingsupon the flat magnet may represent the spaces onthe spectrum.


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52 SIR ISAAC NEWTON.When a beam of white light is decomposed into

the seven different colours of the spectrum, anyparticular colour, when once separated from therest, is not susceptible of any change, or fartherdecomposition, whether it is refracted through prismsor reflected from mirrors. It may become fainteror brighter, but Newton never could, by any process,liter its colour or its refrangibility.Among the various bodies which act upon light,it is conceivable that there might have been somewhich acted least upon the violet rays and mostupon the red rays. Newton, however, found that

this never took place ; but that the same degree ofrefrangibility always belonged to the same colour,and the same colour to the same degree of refran-gibility.Having thus determined that the seven differentcolours of the spectrum were original or simple, hewas led to the conclusion that whiteness or whitelight is a compound of all the seven colours of thespectrum, in the proportions in which they are rep-resented in Jig. 4. In order to prove this, or whatis called the recomposition of white light out of theseven colours, he employed three different methods.When the beam RR was sepltated into its ele-

mentary colours by the prism ABC, he received the


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RECOMPOSIT10N Of LIGHT. 53Colours on another prism BOB', held either close tothe first or a little behind it, and by the opposite re-fraction of this prism they were all refracted backinto a beam of white light BW, which formed awhite circular image on the wall at W, similar towhat took place before any of the prisms wereplaced in its way.The other method of recomposing white lightconsisted in making the spectrum fall upon a lens atsome distance from it. When a sheet of whitepaper was held behind the lens, and removed to aproper distance, the colours were all refracted intoa circular spot, and so blended as to reproduce lightso perfectly white as not to differ sensibly from thedirect light of the sun.The last method of recomposing white light wasone more suited^to vulgar apprehension. It con-sisted in attempting to compound a white by mix-

ing the coloured powders used by painters. Hewas aware that such colours, from their very nature,could not compose a pure white ; but even this im-perfection in the experiment he removed by an in-genious device. Me accordingly mixed one part ofred lead, four parts of blue bice, and a proper propor-tion of orpiment and verdigris. This mixture wasdun, like wood newly cut, or like the human skin.He now took one-third of the mixture and rubbedit thickly on the floor of his room, where the sunshone upon it through the opened casement, and be-side it, in the shadow, he laid a piece of white paperof the same size. " Then going from them to thedistance of twelve or eighteen feet, so that he couldnot discern the unevenness of the surface of thepowder nor the little shadows let fall from the grittyparticles thereof; the powder appeared intenselywhite, so as to transcend even the paper itself inwhiteness." By adjusting the relative illuminationof the powders and the paper, he was able tomake them both appear of the very same degree 01E2


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64 SIR ISAAC NEWTON.whiteness. "For," says he, "when I was tryingthis, a friend coming to visit me, I stopped him atthe door, and before I told him what the colourswere, or what I was doing, I asked him which of thetwo whites were the best, and wherein they differed?And after he had at that distance viewed them well, heanswered, that they were both good whites, and thathe could not say which was best, nor wherein theircolours differed." Hence Newton inferred thatperfect whiteness may be compounded of differentcolours.As all the various shades of colour which appearin the material world can be imitated by intercept-ing certain rays in the spectrum, and uniting all therest, and as bodies always appear of the same colouras the

lightin which they are placed, he concluded,that the colours of natural bodies are not qualitiesinherent in the bodies themselves, but arise from the

disposition of the particles of each body to stop orabsorb certain rays, and thus to reflect more copiouslythe rays which are not thus absorbed.No sooner were these discoveries given to theworld than theywere opposed with a degree of viru-lence and ignorance which have seldom been com-bined in scientific controversy. Unfortunately forNewton, the Royal Society contained few individualsof pre-eminent talent capable of appreciating thetruth of his discoveries, and of protecting him againstthe shafts of his envious and ignorant assailants.This eminent body, while they held his labours inthe highest esteem, were still of opinion that hisdiscoveries were fair subjects of discussion, andtheir secretary accordingly communicated to him allthe papers which were written in opposition to hisviews. The first of these was by a Jesuit namedIgnatius Pardies, Professor of Mathematics at Cler-mont, who pretended that the elongation of the sun'simage arose from the inequal incidence of the dif-ferent rays on the first face of the prism, although

vvlebi


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CONTROVERSY WITH LINUS. 65Newton Tiad demonstrated in his own discourse thatthis was not the case. In April, 1672, Newton trans-mitted to Oldenburg a decisive reply to the animad-versions of Pardies ; but, unwilling to be vanquished,this disciple of Descartes took up a fresh position,and maintained that the elongation of the spectrum/might be explained by the diffusion of light on thehypothesis of Grimaldi, or by the diffusion of undu-lations on the hypothesis of Hook. Newton againreplied to these feeble reasonings ; but he contentedhimself with reiterating 1 his original experiments,and confirming them by more popular arguments,and the vanquished Jesuit wisely quitted the field.Another combatant soon sprung up in the personof one Francis Linus, a pt^sician in Liege,* who,on the 6th October, 1674, addressed a letter to afriend in London, containing animadversions onNewton's doctrine of colours. He boldly affirms,that in a perfectly clear sky the image of the sunmade by a prism is never elongated, and that thespectrum observed by Newton was not formed bythe true sunbeams, but by rays proceeding fromsome bright cloud. In support of these assertions,he appeals to frequently repeated experiments onthe refractions and reflections of light which hehad exhibited thirty years before to Sir KenelmDigby, " who took notes upon them ;" and he un-blushingly states, that, if Newton had used thesame industry as he did, he would never have" taken so impossible a task in hand, as to explainthe difference between the length and breadth ofthe spectrum by the received laws of refraction."When this letter was shown to Newton, he refused* This gentleman was the author of a paper in the Philosophical

Transactions, entitled " Optical Assertions concerning the Rainbow."How such a paper could be published by so learned a body seems in thepresent day utterly incomprehensible. The dials which Linus erectedat Liege, and which were the originals of those formerly in the PrioryGardens in London, are noticed in the Philosophical Transactions for1703. In one of them the hours were distinguished by touch.


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56 SIR ISAAC NEWTON.to answer it ; but a letter was sent to Linus referringhim to the answer to Pardies, and assuring him thatthe experiments on the spectrum were made whenthere was no bright cloud in the heavens. Thisreply, however, did not satisfy the Dutch experi-mentalist. On the 25th February, 1675, he addressedanother letter to his friend, in which he gravelyattempts to prove that the experiment of Newtonwas not made in a clear day ; that the prism wasnot close to the hole, and that the length of thespectrum was not perpendicular, or parallel to thelength of the prism. Such assertions could not butirritate even the patient mind of Newton. He morethan once declined the earnest request of Oldenburgto answer these observations; he stated, that, asthe dispute referred to matters of fact, it could onlybe decided before competent witnesses, and hereferred to the testimony of those who had seenhis experiments. The entreaties of Oldenburg, how-ever, prevailed over his own better judgment, and,"lest Mr. Linus should make the more stir," thisgreat man was compelled to draw up a long andexplanatory reply to reasonings utterly contempti-ble, and to assertions altogether unfounded. Thisanswer, dated November 13th, 1675, could scarcelyhave been perused by Linus, who was dead on the15th December, when his pupil Mr. Gascoigne, tookup the gauntlet, and declared that Linus had shownto various persons in Liege the experiment whichproved the spectrum to be circular, and that SirIsaac could not be more confident on his sidethan they were on the other. He admitted, how-ever, that the different results might arise from dif-ferent ways of placing the prism. Pleased with the" handsome genius of Mr. Gascoigne's letter," Newton replied even to it, and suggested that the spec-trum seen by Linus may have been the circular oneformed by one reflexion, or, what he thought moreprobable, the circular one formed by two refraction^


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CONTROVERSY WITH LUCAS. 57and one intervening reflection from the base of theprism, which would be coloured if the prism was

| not an isosceles o^d. This suggestion seems tohave enlightened the Dutch philosophers. Mr. Gas-coigne, having no conveniences for making the ex-

( periments pointed out by Newton, requested Mr.Lucas of Liege to perform them in his own house.This ingenious individual, whose paper gave greatsatisfaction to Newton, and deserves the highestpraise, confirmed the leading results of the Englishi philosopher ; but though the refracting angle of hisprism was 60 and the refractions equal, he nevercould obtain a spectrum whose length was morethar. from three to three and a half times its breadth,while Newton found the length to be Jive times itsbreadth. In our author's reply, he directs his atten-tion principally to this point of difference. Herepeated his measures with each of the three anglesof three different prisms, and he affirmed that Mr.Lucas might make sure to find the image as long orlonger than he had yet done, by taking a prism withplain surfaces, and with an angle of 66 or 67.He admitted that the smallness of the angle in Mr.Lucas's prism, viz. 60, did not account for theshortness of the spectrum which he obtained withit ; and he observed in one of his own prisms thatthe length of the image was greater in proportionto the refracting angle than it should have been ; aneffect which he ascribes to its having a greaterrefractive power. There is every reason to believethat the prism of Lucas had actually a less disper-sive power than that of Newton ; and had the Dutchphilosopher measured its refractive power insteadof guessing it, or had Newton been less confidentthan he was* that all other prisms must give a* Newton speaks with singular positiveness on this subject. " For /know" says he, "that Mr. Lucas's observations cannot hold where therefracting angle of the prism is full 60, and the day is clear, and thefall length of the colours is measured, and the breadth of the imageanswers to the sun's diameter : and seeing I am well assured of th*


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58 SIR ISAAC NEWTON.spectrum of the same length as his in relation to itsrefracting angle and its index of refraction, the in-vention of the achromatic telescope would havebeen the necessary result. The objections of Lucasdrove our author to experiments which he had neverbefore made, to measure accurately the lengths ofthe spectra with different prisms of different anglesand different refractive powers ; and had the Dutchphilosopher maintained his position with more ob-stinacy, he would have conferred a distinguishedfavour upon science, and would have rewardedNewton for all the vexation which had sprung fromthe minute discussion of his optical experiments.Such was the termination of his disputes with theDutch philosophers, and it can scarcely be doubtedthat it cost him more trouble to detect the origin ofhis adversaries' blunders, than to establish the greattruths which they had attempted to overturn.

Harassing as such a controversy must have beento a philosopher like Newton, yet it did not touchthose deep-seated feelings which characterize thenoble and generous mind. No rival jealousy yetpointed the arguments of his opponents ; no chargesof plagiarism were yet directed against his personalcharacter. These aggravations of scientific contro-versy, however, he was destined to endure ; and inthe dispute which he was called to maintain bothagainst Hooke and Huygens, the agreeable con-sciousness of grappling with men of kindred powerswas painfully imbittered by the personality andjealousy with which it was conducted.Dr. Robert Hooke was about seven years olderthan Newton, and was one of the ninety-eightoriginal or unelected members of the Royal Society.,truth and exactness of my own observations, I shall be unwilling to fa'diverted by any other experiments from having a fair end made of thisin the first place." On the supposition that his prism was one of very;low dispersive power, Mr. Lucas might, with perfect truth, have usedithe very same language towards Newton.


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CHARACTER OF DR. HOOKE. 59He possessed great versatility of talent, yet, thoughhis genius was of the most original cast, and hisacquirements extensive, he had not devoted himselfwith fixed purpose to any particular branch ofknowledge. His numerous and ingenious inven-tions, of which it is impossible to speak too highly,gave to his studies a practical turn which unfittedhim for that continuous labour which physical re-searches so imperiously demand. The subjectsof light, however, and of gravitation seem to havedeeply occupied his thoughts before Newton ap-peared in the same field, and there can be no doubtthat he had made considerable progress in both ofthese inquiries. With a mind less divergent in itsEursuits,

and more endowed with patience of thought,e might have unveiled the mysteries in which boththese subjects were enveloped, and preoccupied theintellectual throne which was destined for his rival ;but the infirm state of his health, the peevishnessof temper which this occasioned, the number ofunfinished inventions from which he looked both

for fortune and fame, and, above all, his inordinatelove of reputation, distracted and broke down theenergies of his powerful intellect. In the morematured inquiries of his rivals he recognised, andoften truly, his own incompleted speculations ; andwhen he saw others reaping the harvest for whichhe had prepared the ground, and of which he hadsown the seeds, it was not easy to suppress themortification which their success inspired. In thehistory of science, it has always been a difficult taskto adjust the rival claims of competitors, when theone was allowed to have completed what the otherwas acknowledged to have begun. He who com-mences an inquiry, and publishes his results, oftengoes much farther than he has announced to theworld, and, pushing his speculations into the veryheart of the subject, frequently submits them to th&ear of friendship. From the pedestal of his pub-


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60 SIR ISAAC NEWTON.lished labours his rival begins his researches, andbrings them to a successful issue ; while he has inreality done nothing more than complete and de-monstrate the imperfect speculations of his prede-cessor. To the world, and to himself, he is nodoubt in the position of the principal discoverer}but there is still some apology for his rival whenhe brings forward his unpublished labours ; and someexcuse for the exercise of personal feeling, when he.measures the speed of his rival by his own proximityto the goal.The conduct of Dr. Hooke would have beenviewed with some such feeling, had not his arro-gance on other occasions checked the natural cur-rent of our sympathy. When Newton presentedhis reflecting telescope to the Royal Society, Dr.Hooke not only criticised the instrument with undueseverity, but announced that he possessed an infal-lible method of perfecting all kinds of optical instru-ments, so that " whatever almost hath been innotion and imagination, or desired in optics, maybe performed with great facility and truth."Hooke had been strongly impressed with thebelief, that light consisted "in the undulations of ahighly elastic medium pervading all bodies; and,guided by his experimental investigation of the phe-nomena of diffraction, he had even announced the

(; great principle of interference, which has performed' such an important part in modern science. Regard-ing himself, therefore, as in possession of the truetheory of light, he examined the discoveries ofNewton in their relation to his own speculativeviews, and, finding that their author was disposed toconsider that element as consisting of material par-ticles, he did not scruple to reject doctrines whichhe believed to be incompatible with truth. Dr. ,Hooke was too accurate an observer not to admit

jthe general correctness of Newton's observations. ;He allowed the existence of different refractions,


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CONTROVERSY WITH HUYGENS. 61the unchangeableness of the simple colours, and the.production of white light by the union of all thepolours of the spectrum ; but he maintained that thepifferent refractions arose from the splitting andrarefying of ethereal pulses, and that there are onlytwo colours in nature, viz. red and violet, which pro-duce by their mixture all the rest, and which arethemselves formed by the two sides of a split pulseor undulation.In reply to these observations, Newton wrote anable letter to Oldenburg, dated June 11, 1672, inwhich he examined with great boldness and forceof argument the various objections of his opponent,and maintained the truth of his doctrine of colours,as independent of the two hypotheses respectingthe origin and production of light. He acknow-ledged his own partiality to the doctrine of themateriality of light ; he pointed out the defects ofthe undulatory theory ; he brought forward new ex-periments in confirmati on of his former results ;and he refuted the opini >ns of Hooke respecting theexistence of only two simple colours. No replywas made to the powe ul arguments of Newton ;and Hooke contented h nself with laying before theSociety his curious ob? vations on the colours ofsoap-bubbles, and of plates of air, and in pursuinghis experiments on the diffraction of light, which,after an interval of two years, he laid Tbefore thesame body.After he had thus silenced the most powerful ofhis adversaries, Newton was again called upon todefend himself against a new enemy. ChristianHuygens, an eminent mathematician and naturalphilosopher, who, like Hooke, had maintained theundulatory theory of light, transmitted to Olden-burg various animadversions on the Newtonian doc-trine ; but though his knowledge of optics was ofthe most extensive kind, yet his objections werenearly as groundless as those of his less enlightenedF


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62 SIR ISAAC NEWTON,countryman. Attached to his own hypothesis re-specting the nature of light, namely, to the systemof undulation, he seems, like Dr. Hooke, to haveregarded the discoveries of Newton as calculated tooverturn it ; but his principal objections related tcthe composition of colours, and particularly of whitelight, which he alleged could be obtained from theunion of two colours, yellow and blue. To this and^similar objections, Newton replied that the coloursin question were not simple yellows and blues, butwere compound colours, in which, together, all thecolours of the spectrum were themselves blended ;and though he evinced some strong traces of feelingat being again put upon his defence, yet his highrespect for Huygens induced him to enter withpatience on a fresh development of his doctrine.Huygens felt the reproof which the tone of thisanswer so gently conveyed, and in writing to Olden-burg, he used the expression, that Mr. Newton" maintained his doctrine with some concern." Tothis our author replied, " As for Mr. Huygens's ex-pression, I confess it was a little ungrateful to me,to meet with objections which had been answeredbefore, without having the least reason given mewhy those answers were insufficient." But thoughHuygens appears in this controversy as a rashobjector to the Newtonian doctrine, it was after-ward the fate of Newton to play a similar partagainst the Dutch philosopher. When Huygenspublished his beautiful law of double refraction inIceland spar, founded on the finest experimentalanalysis of the phenomena, though presented as aresult of the undulatory system, Newton not onlyrejected it, but substituted for it another law entirelyinconsistent with the experiments of Huygens, whichNewton himself had praised, and with those of allsucceeding philosophers.The influence of these controversies on the mindof Newton seems to have been highly exciting


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REFRACTING TELESCOPES. 63Even the satisfaction of humbling all his antago-nists he did not feel as a sufficient compensationfor the disturbance of his tranquillity.

" 1 intend,"says he,* " to be no farther solicitous about mattersof philosophy. And therefore I hope you will nottake it ill if you find me never doing any thing morein that kind ; or rather that you will favour me inmy determination, by preventing, so far as you canconveniently, any objections or other philosophicalletters that may concern me." In a subsequent let-ter in 1675, he says, " I had some thoughts of writinga further discourse about colours, to be read at oneof your assemblies ; but find it yet against the grainto put pen to paper any more on that subject ;" andin a letter to Leibnitz, dated December the 9th, 1675,lie observes,

" I was so persectited with discussionsarising from the publication of my theory of light,that I blamed my own imprudence for parting withso substantial a blessing as my quiet to run after ashadow."

CHAPTER V.Mistake ofNewton in supposing that the Improvement nf Refracting

Telescopes was hopeless Mr. Hall invents the Achromatic TelescopePrinciples of the Achromatic Telescope explained // is re-inventedby Dollond, and improved by future Artists Dr. Blair's Aplanatie

Telescope Mistakes in Newton's Analysis of the Spectrum ModernDiscoveries respecting the Structure of the Spectrum.THE new doctrines of the composition of light,and of the difTerent refrangibility of the rays whichcompose it, having been thus established upon animpregnable basis, it will be interesting to take a

general view of the changes which they have under-I etter to Oldenburg in 1672, containing his first reply to Huygens.


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64 SIR ISAAC NEWTON.gone since the time of Newton, and of their influ-ence on the progress of optical discovery.There is no fact in the history of science moresingular than that Newton should have believedthat all bodies produced spectra of equal length, orseparated the red and violet rays to equal distanceswhen the refraction of the mean rays was thesame. This opinion, unsupported by experiments,and not even sanctioned by any theoretical views,seems to have been impressed upon his mind withall the force of an axiom.* Even the shortness ofthe spectrum observed by Lucas did not rouse himto further inquiry ; and when, under the influenceof this blind conviction he pronounced the improve-ment of the refracting telescope to be desperate, hechecked for a long time the progress of this branchof science, and furnished to future philosophers alesson which cannot be too deeply studied.In 1729, about two years after the death of SirIsaac, an individual unknown to science broke thespell in which the subject of the spectrum had beenso singularly bound. Mr. Chester More Hall, ofMore Hall in Essex, while studying the mechanismof the human eye, was led to suppose that tele-scopes might be improved by a combination oflenses of different refractive powers, and he actuallycompleted several object-glasses upon this principle.The steps by which he arrived at such a construc-tion have not been recorded

; but it is obvious thathe must have discovered what escaped the sagacityof Newton, that prisms made of different kinds of* In an experiment made by Newton, he had occasion to counteractthe refraction of a prism of glass by another prism of water ; and hadhe completed the experiment, and studied the result of it, he could nothave failed to observe a quantity of uncorrected colour, which would

haTO led him to the discovery of the different dispersive powers of bodies.But in order to increase the refractive power of the water, he mixedwith it a little sugar of lead, the high dispersive power of which seemsto have rendered the dispersive power of the water equal to that of iheglass, and thus to have corrected the uncompensated colour of the glassprism.


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ACHROMATIC TELESCOPE. 66produced different degrees of separation of thered and violet rays, or gave spectra of different

lengths when the refraction of the middle ray of thespectrum was the same.In order to explain how such a property led himto the construction of a telescope without colour, oran achromatic telescope, let us take a lens LL ofcrown or plate glass, whose focal length LY isabout twelve inches. When the sun's rays SL,Fig. 6.

SL fall upon it, the red will be refracted to R, theyellow to Y, and the violet to V. If we now placebehind it a concave lens II of the same glass, andof the same focus or curvature, it will be found,both by experiment and by drawing the refractedrays, according to the rules given in elementaryworks, that the concave glass // will refract therays LR, LR into LS', LS ; , and the rays LV, LVinto LS', LS' free of all colour; but as these rayswill be parallel, the two lenses will not have afocus, and consequently cannot form an image soas to be used as the object-glass of a telescope.This is obvious from another consideration ; forsince the curvatures of the convex and concavelenses are the same, the two put together will beexactly the same as if they were formed out of asingle piece of glass, having parallel surfaces like awatch-glass, so that the parallel rays of light SL,F2


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66 SIR ISAAC NEWTON.SL will pass on in the same direction LS', LS*affected by equal and opposite refractions as in apiece of plane glass.Now, since the convex lens LL separated thewhite light SL, SL into its component colouredrays, LV, LV being the extreme violet, and LRLR the extreme red ; it follows that a similar con-cave lens of the same glass is capable of unitinginto white light LS', LS' rays, as much separatedas LV, LR are. Consequently, if we take a con-cave lens II of the same, or of a greater refractivepower than the convex one, and having the powerof uniting rays farther separated than LV, LR are,a less concavity in the lens II will be sufficient tounite the rays LV, LR into a white ray LS' ; butas the lens // is now less concave than the lens LLis convex, the concavity will predominate, and theuncoloured rays LS', LS' will no longer be parallel,but will converge to some point 0, where they willform a colourless or achromatic image of the sun.The effect now described may be obtained bymaking the convex lens LL of crown or of plateglass, and the concave one of flint glass, or that ofwhich wineglasses are made. If the concave lens// has a greater refractive power than LL, which isalways the case, the only effect of it will be tomake the rays converge to a focus more remotethan O, or to render a less curvature necessary in//, if O is fixed for the focus of the combined lenses.Such is the principle of the achromatic telescopeas constructed by Mr. Hall. This ingenious indi-vidual employed working opticians to grind hislenses, and he furnished them with the radii of thesurfaces, which were adjusted to correct the aber-ration of figure as well as of colour. His invention,therefore, was not an accidental combination of aconvex and a concave lens of different kinds ofglass, which might have oeen made merely for ex-periment; but it was a complete achromatic tele-


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ACHROMATIC TELESCOPES. 67scope, founded on a thorough knowledge of thelifferent dispersive powers of crown and flint glass.[t is a curious circumstance, however, in the his-;ory of the telescope, that this invention was ac-tually lost. Mr. Hall never published any account(}f his labours, and it is probable that he kept themsecret till he should be able to present his instru-nent to the public in a more perfect form ; and it,vas not till John Dollond had discovered the pro-perty of light upon which the instrument depends,md had actually constructed many fine telescopes,;hat the previous labours of Mr. Hall were laid be-bre the public.* From this period the achromaticelescope underwent gradual improvement, and byhe successive labours of Dollond, Ramsden, Blair,Pulley, Guinand, Lerebours, and Fraunhofer, it has>ecome one of the most valuable instruments in.)hysical science.Although the achromatic telescope, as constructed

>y Dollond, was founded on the principle that thespectra formed by crown and flint glass differedmly in their relative lengths, when the refraction)f the mean ray was the same, yet by a more mi-lute examination of the best instruments, it wasbund that they exhibited white or luminous objectsinged on one side with a green fringe, and on theHher with one of a claret colour. These colours,tvhich did not arise from any defect of skill in the.rtist, were found to arise from a difference in theextent of the coloured spaces in two equal spectra'ormed by crown and by flint glass. This propertywas called the irrationality of the coloured spaces,and the uncorrected colours which remained when;he primary spectrum of the crown glass was cor*rected by the primary spectrum of the flint glasswere called the secondary or residual spectrum. By* See the article OPTICS in the Edinburgh Encyclopedia, vol xv479, note.


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68 SIR ISAAC NEWTON.a happy contrivance, which it would be out of placihere to describe, Dr. Blair succeeded in correctingthis secondary spectrum, or in removing the greerand claret-coloured fringes which appeared in th


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PRISMATIC SPECTRUM. 69withstanding the greater diminution of the sun's

[apparent diameter* It may now be asked, which ofall these spectra are we to consider as exhibitingthe number, and arrangement, and extent of thecoloured spaces proper to be adopted as the true[analysis of a solar ray.The spectrum observed by Newton has surely noclaim to our notice, merely because it was observedupon the surface of the earth. The spectrum ob-tained in Mercury affords no analysis at all of theincident beam, the colours being almost all com-pound, and not homogeneous, and that of Newtonis liable to the same objection. Had Newton ex-amined his spectrum under the very same circum-stances in winter and in summer, he would havefound the analysis of the beam more complete insummer, on account of the diminution of the sun'sdiameter ; and, therefore, we are entitled to say that ineither the number nor the extent of the coloured/spaces, as given by Newton, are those which belong!fohomogeneous and uncompoundedjight. [

'I'tie spectrum obtained in Jupiter and Saturn isthe only one where the analysis is complete, as it isincapable of having its character altered by any far-ther diminution of the sun's diameter. Hence weare forced to conclude, not only that the numberand extent of the primitive homogeneous colours,as given by Newton, are incorrect ; but that if he hadattempted to analyze some of the primitive tints inthe spectrum, he would have found them decidedlycomposed of heterogeneous rays. There is oneconsequence of these observations which is some-what interesting. A rainbow formed in summer,when the sun's diameter is least, must have its col-ours more condensed and homogeneous than in win-ter, when the size of its disk is a maximum, andwhen the upper or the under limb of the sun iseclipsed, a rainbow formed at that time will loseentirely the yellow rays, and have the green and the


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70 SIR ISAAC NEWTON.red in perfect contact. For the same reason, a rain- ]bow formed in Venus and Mercury will be destituteof green rays, and have a brilliant bow of whitelight separating two coloured arches ; while in MarsJupiter, Saturn, and the Georgian planet, the bo\\will exhibit only four homogeneous colours.From his analysis of the solar spectrum, Newtorconcluded, " that to the same degree of refrangibilit}ever belonged the same colour, and to the samecolour ever belonged the same degree of refrangi-bility ;" and hence he inferred, that red, orange, yel-low, green, blue, indigo, and violet were primary and.simple colours. He admitted, indeed, that "thesame colours in specie with these primary ones ma);be also produced by composition. For a mixture ofyellow and blue makes green, and of red and yelloumakes orange;" but such compound colours wereeasily distinguished from the simple colours of thespectrum by the circumstance, that they are alwayscapable of being resolved by the action of the prisminto the two colours which compose them.This view of the composition of the spectrummight have long remained unchallenged, had we notbeen able to apply to it a new mode of analysis.Though we cannot separate the green rays of thespectrum into yellow and blue by the refraction ofgrisms, yet

if we possessed any substance whichad a specific attraction for blue rays, and whichstopped them in their course, and allowed the yel-low rays to pass, we should thus analyze the greenas effectually as if they were separated by refraction.The substance which possesses this property is apurplish blue glass, similar to that of which finger-glasses are made. When we view through a pieceof this glass, about the twentieth of an inch thick,a brilliant prismatic spectrum, we find that it hasexercised a most extraordinary absorptive action onthe different colours which compose it. The redpart of the spectrum is divided into two red spaces^


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SPECTRUM OF THREE COLOURS. 71separated by an interval entirely devoid of light.

e Next to the inner red space comes a space of brightoi/ellow, separated from the red by a visible interval.;,A.fter the yellow comes the gr&m, with an obscure

rjspace between them, then follows the blue and thebiolet, the last of which has suffered little or noDiminution. Now it is very obvious, that in thisJbxperiment, the blue glass has actually absorbedphe red rays, which, when mixed with the yellowan one side, constituted orange, and the blue rays,which, when mixed with the yellow on the otherside, constituted green, so that the insulation of;he yellow rays thus effected, and the disappearance)f the orange, and of the greater part of the greenlight, proves beyond a doubt that the orange andrreen colours in the spectrum are compound col->urs, the former consisting of red and yellow rays,md the latter of yellow and blue rays of the veryame refrangibility. If we compare the two redspaces of the spectrum seen through the blue glasskvith the red space seen without the blue glass, itvill be obvious that the red has experienced suchin alteration in its tint by the action of the blueflass, as would be effected by the absorption of aimall portion of yellow rays ; and hence we con-dude, that the red of the spectrum contains a slightinge of yellow, and that the yellow space extends>ver more than one-half of the spectrum, includinghe red, orange, yellow, green, and blue spaces.I have found also that red light exists in the yel-ow space, and it is certain that in the violet spaceed light exists in a state of combination with the

)lue rays. From these and other facts which itivould be out of place here to explain, I concludehat the prismatic spectrum consists of three differ-nt spectra, viz. red, yellow, and blue, all havinghe same length, and all overlapping each other.Hence red, yellow, and blue rays of the very sameefrangibility coexist at every point of the spec*


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72 em ISAAC NEWTON.trum ; but the colour at any one point will be thaiof the predominant ray, and will depend upon therelative distance of the point from the maximumordinate of the curve which represents the intensityof the light of each of the three spectra.This structure of the spectrum, which harmo-nizes with the old hypothesis of three simple colours,will be understood from the annexed diagramwhere MN is the spectrum of seven colours, alcompounded of the three simple ones, red, yellow

Fig. 7.

and blue. The ordinates of the curves R, Y, ancB will express the intensities of each colour at dif-ferent points of the spectrum. At the red extrem-ity M of the spectrum, the pure red is scarcel}altered by the very slight intermixture of yellowand blue. Farther on in the red space, the yelloubegins to make the red incline to scarlet. It therexists in sufficient quantity to form orange, and, asthe red declines, the yellow predominates over thefeeble portion of red and blue which are mixed wit!it. As the yellow decreases in intensity, the in-creasing blue forms with it a good green, and theblue rising to its maximum speedily overpowers thesmall portion of yellow and red. When the bluebecomes very faint, the red exhibits its influencein converting it into violet, and the yellow ceasei


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LIXES IN THE SPECTRtTM. 73to exercise a marked influence on the tint. Theinfluence of the red over the blue space is scarcelyperceptible, on account of the great intensity of theblue light ; but we may easily conceive it to reap-pear and form the violet light, not only from therapid decline of the blue light, but from the greaterinfluence of the red rays upon the retina.These views may, perhaps, be more clearly under-stood by supposing that a certain portion of whitelight is actually formed at every point of the spec-trum by the union of the . requisite number of thethree coloured rays that exist at any point. Thewhite light thus formed will add to the brilliancywithout affecting the tint of the predominant colour.In the violet space we may conceive the smallportion of yellow which exists there to form whitelight with a part of the blue and a part of the red,so that the resulting tint will be violet, composedof the blue and the small remaining portion of red,mixed with the white light. This white light willpossess the remarkable property of not being sus-ceptible of decomposition by the analysis of theprism, as it is composed of red, yellow, and bluerays of the very same refrangibility. The insula-tion of this white light by the absorption of thepredominant colours I have effected in the green,yellow, and red spaces, and by the use of new ab-sorbing media we may yet hope to exhibit it in someof the other colours, particularly in the brightestpart of the blue space, where an obvious approxi-mation to it takes place.: Among the most important modern discoveriesrespecting the spectrum we must enumerate thatof fixed dark and coloured lines, which we owe tothe sagacity of Dr. Wollaston and M. Fraunhofer.Two or three of these lines were discovered by Dr.Wollaston, but nearly 600 have been detected bymeans of the fine prisms and the magnificent appa-ratus of the Bavarian optician. These lines areG


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SIR ISAAC NEWTOW.parallel to one another, and perpendicular to thelength of the spectrum. The largest occupy aspace from 5" to 10" in breadth. Sometimes theyoccur in well-defined lines, and at other times ingroups ; and in all spectra formed from solar light,they preserve the same order and intensity, and thesame relative position to the coloured spaces, what-ever be the nature of the prism by which they arefroduced. Hence these lines are fixed points, bywhich the relative dispersive powers of differentmedia may be ascertained with a degree of accu-racy hitherto unknown in this branch of science. Inthe light of the fixed stars, and in that of artificialflames, a different system of lines is produced, andthis system remains unaltered, whatever be the na-ture of the prism by which the spectrum is formed.The most important fixed lines in the spectrumformed by light emitted from the sun, whether it isreflected from the sky, the clouds, or the moon, maybe easily seen by looking at a narrow slit in thewindow-shutter of a dark room, through a hollowprism formed of plates of parallel glass, and filledwith any fluid of a considerable dispersive power.The slit should not greatly exceed the twentieth ofan inch, and the eye should look through the thinnestedge of the prism where there is the least thicknessof fluid. These lines I have found to be the boun-daries of spaces within which the rays have par-ticular affinities for particular bodies.


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COLOURS OF THIN PLATES. 75

CHAPTER VLColours of thin Plates first studied by Boy ^and Hooke Newton de-termines the Law of their Production His Theory of Fits of EasyRefection and Transmission Colours of thick Plates.IN examining the nature and origin of colours asthe component parts of white light, the attention ofNewton was directed to the curious subject of thecolours of thin plates, and to its application to ex-

plain the colours of natural bodies. His earliestresearches on this subject were communicated, inhis Discourse on Light and Colours, to the RoyalSociety, on the 9th December, 1675, and were readat subsequent meetings of that body. This discoursecontained fuller details respecting the compositionand decomposition of light than he had given in hisletter to Oldenburg, and was concluded with ninepropositions, showing how the colours of thin trans-parent plates stand related to those of all naturalbodies.The colours of thin plates seem to have beenfirst observed by Mr. Boyle. Dr. Hooke afterwardstudied them with some care, and gave a correctaccount of the leading phenomena, as exhibited inthe coloured rings upon soap-bubbles, and betweenplates of glass pressed together. He recognisedthat the colour depended upon some certain thick-ness of the transparent plate, but he acknowledgesthat he had attempted in vain to discover the rela-tion between the thickness of the plate and thecolour which it produc

Life of Sir Isaac Newton 1840

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