67

COMMITTEE ON SCIENCE AND THE ARTS.

Report o~ the CaloTtric, Dioptric, and Catadioptric Systems of Lights fo~" Lighthouses.

The Conamittee on Science and the Arts, constituted by the Franklin Institute of the State of Pennsylvania, for the Promotion of the Mechanic Arts, in compliance with the follow- ing resolution, passed by the Franklin Institute at their meeting held April 19th, 1849, viz: ,,Resolved, That the Committee on Science and the Arts be requested to examine and repOrt to the next meeting of the Institute, their opinion of the relative value of the present

Catoptrie System adopted in the United States, compared with the Fresnel Dioptric and Catadioptric System established in the Lighthouses in Europe and elsewhere, so far as the same can be obtained from documents in the possession of the Institute," RrPOnT:--

• That they have examined the subject intrusted to them with the care and attention which its great importance demands. The im- mense amount of property, and the many valuable lives dependent on the proper illumination of coasts and harbors, have attracted the attention of men of science, and of nearly all civilized governments. The results have been witnessed in the very great improvements in the structure of light- houses, and in their reflecting and refraetingapparatus, especially ofthelatter.

The Committee have studied the voluminous documents containing not merely the opinions of a number of the most scientific men in Europe and America, but the actual practical results described in official statements of Lighthouse Boards, in every country where lights of high character have been established. They also have examined much voluminous corres- pondence, and have endeavored to avail themselves of every attainable source of information relating to the subject. They have also examined the Dioptric and Catadioptrie Apparatus of Fresnel, as displayed in the four orders exhibited before the Franklin Institute.

The Catoptric System adopted in the United States is a very imperfect imitation of the European method, which was in use long before its alleged invention in this Union. Paraboloidal reflectors, with lamps placed in their foei, were in use nearly seventy years since, in France and Great Britain. The addition of a convex lens in front of the lamp, placed in the focal point of such a reflector, could only have been suggested by a person entirely ignorant of the elementary principles of optics.*

We need not, however, at present enter into the history of the lighthouse system in the United States. We are gratified to learn that a few improve- ments have been made by the introduction of better forms of reflectors, and in their adjuncts, as well as in their size, finish, and mountings, but these few eases are merely exceptions to the general rule; and it must be ac- knowledged that the Catoptrie System of this country is, in actual practice, farinferior to the Catoptric System of Europe. I -[ We do not deem the former to be of sufficient value to contrast with the Dioptric and Catadioptrie Sys- tems of Fresnel, which is rapidly superseding even the latter (the reflecting system) in its best condition. We therefore limit our comparison to the reflecting apparatus of Europe and the apparatus of Fresnel.

A paraboloidal reflector of the best description receives but two-thirds * The small spherical reflectors are also defective in principle, but it is not necessary to

discuss the defects of this antique apparatus. I" These numbers refer to articles in the Appendix, which will be found in the next number.

68 Franklin Institute.

of the light emitted from the lamp; half of this only is reflected, equal to one-third of the whole, 2 and of this third a portion is intercepted by the in- tervention of a part of the lamp, diminishing the available light to less than one-third--perhaps to one-fourth; and of this one-fourth, a considerable portion is, from the divergence of the penumbral cone, too faint to produce much effect at great distances. In fixed lights many such reflectors are arranged in one or more circles or arcs; but even when very numerous~ viz. from twenty to thirty, the light is very unequally distributed, and in some azimuths is invisible; when the reflectors are less numerous~ viz. from ten to twenty, this defect is still more manifest. 3

With the refracting and Catadibptric apparatus of Fresnel for fixed lights~ the whole circle is equally illuminated, (especially when the latest improve- ments have been introduced,) 4 and nine-tenths nearly of the light emitted from the lamp is rendered available ~* ~--glass, as a refracting material in this system, possessing the same advantage over reflectors as the refracting tele- scope possesses over the reflecting species. The consumption of oil to produce an equal light might readily be inferred to be much less, and prac- tice has proved beyond a doubt that such is the fact. 6

The superiority })f the revolving and of the flashing lights of Fresnel over the revolving reflecting lights must also be conceded, q the superior brilliancy of these lights, the intense concentration of their rays, conse- quently their penetrating power, as well as their economy in the consump- tion ofoil, being manifest to the Committee, it remains to inquire whether their first cost or the expense of their management be greater.

In Europe, f o r a light o f equal intensity, the first cost is not greater, but as in practice the Fresnel lights are much more intense than the reflecting lights, the .first cost of the apparatus is, in a small ratio, greater than that of the latter, when of the best description, s (of course we do not allude to the imperfect apparatus in this country,) but their great economy in oil is more than a compensation for the small additional prime cost. 9 They do not require more persons to attend to them, although, in fact, more are required in . . . . . Europe, for both reflecting and refracting.gli hts,. than in. the reflecting. hghts m the Umted States, but this increase is entirely owing to the lm- pnrtance there justly attached to the maintenance of a steady constant light, which cannot be insured in either syste m when one keeper only is em- ployed. ~ o

The Committee are strongly impressed with the great excellence of the Fresnel system,~a system which has already been established in nearly three hundred places (and everywhere with satisfaction) in Europe, where it is gradually superseding the old and imperfect Catoptric lights, whilst in no instance has it been abandoned after it has been established.

The opinion of scientific men throughout the world is believed to be decidedly in favor of the Fresnel system of lights, where a large arc of a circle requires illumination. In the United States, this invention has been introduced only at one station. The Committee earnestly hope that our whole coast, wherever it may be adapted to them, on both oze~hs, and on the lakes, ,nay speedily be provided with this briiliant, economical,'and admirably devised gift of the science and art of the nineteenth century

By order of the Committee, WILLIA.3,I H.A.MILTON, ./tctuary. Philadelphia, .}lay l Oth, 1849.

~* Subject, of course, to a small deduction in the passage through the glas~