BERGER, GRAHAM, AND HSIA

in fusion frequency, as compared to normals, in char-acteristic spectral regions. Similar findings had pre-viously been published by Ferry.27 Deuteranopesshowed decreased fusion frequencies in the green, blue,and yellow regions of the spectrum. These results are inaccord with those of the present study.

27 E. S. Ferry, Am. J. Sci. 44, 192 (1892).

ACKNOWLEDGMENTS

We wish to express our appreciation to ProfessorC. G. Mueller, Professor Carney Landis, and Mr.Aaron Hyman for valuable critical suggestions. Specialthanks are due our subject for the many hours thatshe spent in observation during the course of theseexperiments.

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA VOLUME 48, NUMBER 9 SEPTEMBER, 1958

Some Visual Functions of a Unilaterally Color-Blind Person. II. BinocularBrightness Matches in Various Spectral Regions*

EDA BERGERJ C. H. GRAHAM, AND YUN HsIADepartment of Psychology, Columbia University, New York, New York

(Received December 23, 1957)

Binocular brightness matches in eight spectral regions, ranging from one having a spectral centroid at452 m to one with a spectral centroid at 681 mu, were determined for a unilaterally dichromaticsubject, a young woman with normal color vision in one eye and basically deuteranopic vision in the other.The measurements were made at photopic luminance levels by means of a polarization photometer in whichthe field of view of each eye subtended 1.8°.

For a report of apparent equality of brightness of the two test fields, the luminance requirements for thefield seen by the subject's color-blind eye exceed those for the field viewed by her normal eye in all but thered spectral regions. The luminance loss varies with wavelength; it is greatest in the green, less in the blue,and even less in the yellow. These selective spectral luminosity losses are maintained over a luminance rangeof approximately 2.5 log millilamberts. These data confirm earlier findings on selective luminosity losses atthreshold for this subject's deuteranopic eye.

INTRODUCTION

IN discussions of the characteristics of color-blindvision, it has generally been asserted that the two

major forms of dichromatism may be distinguished bythe fact that protanopes show decreased sensitivity toradiations from the long wave end of the spectrum,whereas deuteranopes do not exhibit spectral sensitivitylosses. Such statements are based primarily on measure-ments of the luminosity functions of these two types ofcolor defective observers. Compared to the normalluminosity curve, the protanopic curve is greatlyreduced in the red end of the spectrum. The luminositycurve of the deuteranope, on the other hand, has usuallybeen described as similar in all essential respects tothat of the normal trichromat.'

In 1947 Hecht and Hsia2 reported that deuteranopesas well as protanopes suffer substantial reductions intheir absolute luminosity functions. More extensive

* This work was supported by a contract between the Office ofNaval Research and Columbia University, and by a grant fromthe Higgins Fund of Columbia University. Reproduction inwhole or in part is permitted for any purpose of the United Statesgovernment.

t Present address: Research Laboratory of Electronics, Massa-chusetts Institute of Technology, Cambridge, Massachusetts.

1 F. H. G. Pitt, G. Brit. Med. Research Council, Spec. Rept.Ser. No. 200 (1935).

2 S. Hecht and Y. Hsia, J. Gen. Physiol. 31, 141 (1947).

work by Hsia and Graham3 in the recent past hasconfirmed this finding. Hsia and Graham investigatedthe luminosity functions of normals and of dichromats.Their data for five protanopes show the expectedreduction in luminosity at the long wavelengths. Inaddition, however, the luminosity curves of sixdeuteranopes exhibit decreased sensitivities in thegreen and blue regions of the spectrum.

Graham and Hsia4 also measured the thresholdluminosity curves for both eyes of a unilaterally color-blind subject, a young woman with primarily deuter-anopic discriminations in her left eye and normal colorvision in her right eye. The two functions were foundto differ in absolute sensitivity values in the greenand blue regions of the spectrum; the sensitivity ofthe normal eye in these regions is considerably greaterthan the sensitivity of the deuteranopic eye. In the redend of the spectrum, the sensitivities of both eyes aresimilar. The data on this subject are thus in line withthe results Hsia and Graham obtained from theircomparison of the luminosity functions of a group ofdeuteranopes with those of a group of normals: deuter-anopes show a loss in sensitivity in the green and blueas compared with normal subjects.

3 Y. Hsia and C. H. Graham, Proc. Natl. Acad. Sci. 43, 1011(1957).

4 C. H. Graham and Y. Hsia, Proc. Natl. Acad. Sci. 44, 46(1958).

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September1958 VISUAL FUNCTIONS OF UNILATERALLY COLOR-BLIND. II 623

In order to determine whether the selective spectralsensitivity losses manifested by the color-blind eye atthreshold would be maintained at high photopicluminance levels, a comparison was undertaken of thebinocular brightness matches made by the unilaterallycolor-blind subject in various spectral regions.

THE SUBJECT

The subject was a young woman, 25 years of age.On the basis of an extended examination involving theadministration of a series of tests designed to detectcolor deficit, it was concluded that her right eye isnormal and that her left eye is primarily deuteranopicbut with certain complications in the violet part of thespectrum. The tests used as well as the test results aredescribed in a previous experiment with this subjectby Berger, Graham, and Hsia.5

APPARATUS

The apparatus consisted of a polarization type ofphotometer. The optical system employed provided aMaxwellian view. A diagram of this apparatus isshown in Fig. 1. A ribbon filament bulb, monitored at18 amperes and 6-volts dc, serves as the light source (B).The light bulb is housed in an asbestos lamp housecontaining a small circular aperture through which theillumination enters the optical system. Light from thesource is rendered parallel by an achromatic lens (LI)placed at focal distance from the source and thenpasses through a rotatable polaroid (P1 ) and filtersinserted in the filter box at F. The beam next passesthrough two semicircular field stops separated by twocircular fixation spots (S). The diameter of eachsemicircle is 0.50 inch. At a distance of 0.055 inchfrom this longer dimension and at a position corre-sponding to its midpoint is located a fixation spot,0.055 inch in diameter. The other half of the configura-tion, the second fixation spot and its associated semi-circle, constitutes a mirror image of the first arrange-ment. The angular subtense of each semicircular field,in terms of its diameter, is 1.80 while the fixation spotsubtends a visual angle of 12 min at the subject's eye.

Behind the semicircular fields are mounted twopolaroids (P2 and P3 ), one for each field. The polari-zation axis of P2 is vertical, that of P3 is horizontal. Alens (L2) serves to bring the image of the filament to afocus at the eye of the observer. A pair of artificialpupils, 2 mm in diameter, is inserted at the ends of twoviewing tubes (E) whose separation corresponds to thesubject's interpupillary distance. To provide stimulation

E R L2 P2 F P Li

4 L- A O i I P3

B FIG. 1. Diagram ofQ the apparatus as de-

scribed in text.

5'Berger, Graham, and Hsia, J. Opt. Soc. Am, 48, 614 (1958).

of each eye by its corresponding semicircle only, a pairof 10 diopter prisms (R) mounted base temporal on anadjustable frame is placed in the optical path. Thefield of view for each eye fills only the prism in thepath to that eye, thus obviating the need for a dividingseptum. Provision is made for changes in refractionwith wavelength by mounting the prisms in an adjust-able frame which permits backward and forwardmovement of the prisms.

Rotation of the movable polaroid (P1 ) is accomplishedby means of a handle connected to it. A pointer attachedto this handle and sliding along a suitably mountedscale, reading in degrees, indicates the amount ofangular rotation. A continuous variation in theluminance of the test field viewed by each eye is thusachieved by moving the rotatable polaroid through900. Because the planes of polarization of the twostationary polaroids (P2 and P 3 ) are mutually perpen-dicular, luminance variations resulting from movingthe rotatable polaroid continuously from one end of thescale to the other are opposite in direction for the twotest fields; that is, as the test field viewed by the righteye, for example, changes from brighter to dimmer,that seen by the left eye simultaneously changes fromdimmer to brighter. At some point, theoretically at450, the two fields are seen as equally bright. Thus, theexperimental arrangement permits two adjacent in-dependently variable test fields, one viewed by eacheye, to be adjusted to equality of brightness.

The color temperature of the source was found to be28480K with a Leeds and Northrup optical pyrometer.The relative visual effectiveness of this source was thencomputed by multiplying the relative spectral energydistribution of a Planckian radiator at 28480 K by therelative spectral sensitivity of the eye (that is, by theluminosity curve). Measurements of the photopicthreshold luminosity curves for both eyes of the subjecthad previously been obtained by Graham and Hsia4 ;therefore, the subject's luminosity curve for her normaleye has been used in these computations.

The various spectral regions investigated in thisstudy were isolated by means of appropriate Comingglass filters, Wratten gelatin-in-glass filters, or combina-tions of these, so chosen that their spectral centroidswere distributed throughout the spectrum between 450and 680 mu at approximately equal wavelengthintervals. The spectral transmission of these filterswas measured on a Beckman spectrophotometer. For agiven color filter or filter combination, the transmittancevalues (Tx) were multiplied, at 10 mg intervals, bythe "white" light curve (E\V\), that is, by the productof the energy distribution for the 28480K color tem-perature of the lamp (E?) and the luminosity curvefor the subject's normal eye (V),). The resulting productcurve (ExVxT)) then represents the relative values ofluminous flux transmitted by a given filter. Thespectral centroid (X) for each color filter or filter

BERGER, GRAHAM, AND HSIA

combination was computed from the relation

E ExVxTx

where all the symbols have the previously assignedmeanings. In the following discussion of results,reference to any specific wavelength means thespectral centroid (calculated for the normal eye)of the spectral region isolated by a specific color filter.

As pointed out above, when the movable polaroidis set at an angle of 450 to the planes of polarizationof the stationary polaroids, the two test fields shouldtheoretically be of equal luminance. Also, of course, themaximum luminances of the two fields should be thesame, except that for one it occurs at a setting of 900,for the other, at a setting of 0. If these conditionsobtain, then the determination of polaroid trans-mittance values for only one of the test fields issufficient, since those for the other field can thenreadily be computed; the second function will be thedifference between unity and the transmittance of thefirst. This ideal was not realized in the present apparatusas shown by empirical measurements of the maximumluminances of the two fields and their respectiveluminances for a 450 setting of the rotatable polaroid.Each field was, therefore, treated independently inmaking luminance determinations. Over the range from0° to 900 on the movable polaroid scale, the absoluteluminance of each test field in white light was measuredat 150 intervals by means of a monocular matchingtechnique. That is, a brightness match was first madebetween the test field and a standard source of thesame area when both of these were viewed monocularly.The luminance of the standard was then measuredwith a Macbeth illuminometer. For each test fieldseparately, the empirical values so obtained agreedclosely with those calculated from the appropriaterelation: I= Io cos2 0 (or I= o sin20) where 0 is the anglebetween the polarization planes of the movable andone of the stationary polaroids, I is the luminance atany angle 0, and Io is the measured maximum luminancefor the field in question. Since the agreement betweenthe empirical and calculated values was close, theabove relations were used in all subsequent compu-tations of the respective luminances of the fields atgiven angular settings, the computation for each testfield being referred to its own maximum luminancevalue.

Additional calibrations with a Farrand electronmultiplier photometer placed at the exit pupil of thetwo eye pieces yielded essentially the same results asobtained with the monocular matching method andthus provided an independent verification of thecorrectness of the measurements made with the lattertechnique.

The maximum luminance for each test field for each

of the colored test lights was computed from the ratioof the sum of the product curve ( ExVx) for theunfiltered lamp to the sum of the respective colorproduct curve ( E\V-xT) for the filtered lamp andluminances of the fields at any given angular settingof the polaroid then calculated with reference to theserespective maxima.

Filters (Bausch and Lomb) with a coating of inconelwere used to vary the brightness of the test lights infixed steps. The transmittance of each of these filterswas measured on a Martens polarization photometer.

PROCEDURE

Testing was conducted in a darkened room. Obser-vation was binocular, a chin rest being used to aid inlocating and stabilizing the position of the subject'seyes with respect to the eye pieces. While makingobservations, the subject wore corrective lenses whichrendered her emmetropic.

The stimulus pattern consisted of two illuminatedsemicircular test fields separated by a narrow darkstrip in the center of which appeared two small illumi-nated fixation points. On looking through the eyepieces, the subject saw the right semicircle with herright eye, the left one with her left eye. The fixationspots were seen binocularly. The subject was instructedto fuse the two fixation spots so that they appeared tobe a single dot. Thus the test configuration appearedas two halves of a circle symmetrically placed on eitherside of a central fixation point.

The subject was asked to equate the two semicirclesin brightness by manipulating the rotatable polaroid.She was to keep the two fixation spots fused whilemaking her settings. This criterion was employed inorder to ensure constant observation conditions. Inauxiliary instructions, the subject was told not to makesettings when the appearance of the test configurationwas other than the standard one defined above, e.g.,when two fixation spots were seen or when there wasany asymmetry of the two test fields, either withrespect to each other or to the fused dot.

Considerable fatigue and difficulty attends suchobservations. It must be kept in mind that settings ofthis type constitute heterochromatic brightness matchesfor this subject, which introduce an additional compli-cating factor over and above those inherent in thisparticular viewing situation. Whereas the colors ofboth fields were identical to a normal eye, they did notappear as such to the two eyes of the subject for anyof the eight spectral regions explored. The greatestdiscrepancy between the two test fields occurred for thefilter combination having a spectral centroid at500 m, which corresponds closely to the subject'sneutral point (502 mg). At this wavelength, the righteye field appeared as blue-green to the normal eye, theleft eye field was seen as bright gray by the deuteranopiceye. Therefore, a number of preliminary sessions was

624 Vol. 48

September1958 VISUAL FUNCTIONS OF

devoted to accustoming the subject to the task andproviding adequate practice in making settings. Onlywhen she could maintain the requisite fusion withoutundue strain and expressed confidence in her settingswas the actual testing begun.

In any one experimental session, binocular brightnessmatches between the two test fields were made for agiven spectral region. Measurements extended over aluminance range of approximately 2.5 log millilamberts,in steps of 0.6-log unit. Three settings were made ateach fixed luminance level, the five selected levels beingpresented in an ascending order of magnitude.

The subject remained for five minutes in the darkenedexperimental room before measurements were com-menced. She was then presented with the lowest of thefive fixed luminance levels and after achieving the properfusion condition, rotated the movable polaroid untilshe was satisfied that the two fields were equated inbrightness. A bracketing procedure was used in makingthe settings. Prior to the next determination, theexperimenter displaced the rotating polaroid to adifferent scale position from the last setting made bythe subject. These changes were made randomly, so thaton successive trials sometimes the right field, sometimesthe left field, appeared initially brighter.

A minimum of four experimental sessions wasdevoted to obtaining the data with any one color filteror filter combination. Testing with various colors wasrandomized and one session was completed for eachof the spectral regions before the next round of sessionsfor all colors was started. At the outset of a sessionwith any given color, the position of the prisms wasadjusted so that the image of each test field wascentered on the appropriate artificial pupil aperture andthe subject reported that both fields appeared uniformlyilluminated and in sharp focus.

The data to be reported are based on a total of 32experimental sessions, four for each of the eight selectedspectral regions.

TABLE I. Luminances required by the right (normal) and left(color-blind), eyes of a unilateral deuteranope for binocularbrightness matches at various wavelengths.

452 mg 475 mAu 500 mA 538 muLog mL Log mL Log mL Log mL

Right Left Right Left Right Left Right Lefteye eye eye eye eye eye eye eye

0.02 0.73 0.05 0.79 0.53 1.22 0.04 1.100.60 1.35 0.64 1.41 1.09 1.84 0.65 1.661.13 1.88 1.18 1.93 1.61 2.38 1.20 2.211.69 2.54 1.80 2.58 2.20 3.03 1.81 2.822.27 3.14 2.38 3.17 2.84 3.61 2.37 3.35

568 my 591 mAz 640 mp 681 myLog mL Log mL Log mL Log mL

Right Left Right Left Right Left Right Lefteye eye eye eye eye eye eye eye

1.01 1.63 0.35 0.75 0.49 0.56 -0.12 -0.071.60 2.24 0.87 1.32 1.09 1.04 0.43 0.492.15 2.76 1.47 1.94 1.67 1.69 1.04 1.102.76 3.42 1.99 2.47 2.19 2.23 1.55 1.663.35 4.01 2.62 3.12 2.87 2.82 2.22 2.25

4.00

W0>Z

z

a]

0-J0

U10

z

z-J

(00

Z1

3.50

300

250

200

1.50

(00

0.50

0.00

0.00 050 100 1.50 2.00 2.50 3.00 350

LOG LUMINANCE (mL)-NORMAL EYE

FIG. 2. Binocular brightness matches of a unilateral deuteranopein various spectral regions. Each datum point represents theluminances of the test fields seen by their respective eyes whenthe fields are equated for brightness by the subject.

RESULTS

The data are summarized in Table I. This table showsthe average luminance in log millilamberts requiredby each eye for a judgment of equality of brightness ofthe two corresponding test fields at each of the variousspectral regions. Each entry is the average of 12measurements made in four separate experimentalsessions.

A graphical representation of these data is providedin Fig. 2 where the average log luminance of the lefttest field is plotted as a function of the log luminanceof the right test field. Each datum point represents theluminance of the respective test fields when these arematched in brightness by the subject. The lines drawnthrough these points are empirically fitted.

The points for 452, 475, and 500 mu cluster so closelytogether that one function has been drawn through allof them; similarly, for the measurements at 640 and681 mhz, one line again adequately represents bothsets of data.

The functions are linear over the luminance rangetested and are parallel to one another. On a plot suchas that in Fig. 2, a 450 diagonal passing through theorigin corresponds to luminance equality of the twotest fields. The data for the reds, 640 and 681 miu, fallon such a line. Those for the other colors also make anangle of 450 with the abscissa but are displaced upwardon the ordinate. The extent of this upward shift varieswith wavelength and provides a direct measure of thediscrepancy between physical and apparent brightnessequality for the different spectral regions.

The functions in Fig. 2 may all be described by therelation: logld= a logI,+logb, where Id is the luminanceof the left field for the dichromatic eye, i,, the corre-sponding luminance of the right field for the normal eye

0 452mM* 475a 500s 5380 568* 5910 640* 681

I I 625UNILATERALLY COLOR-BLIND.

BERGER, GRAHAM, AND HSIA

TABLE II. Average value by which log luminance requirementsfor the color-blind eye exceed corresponding log luminances forthe normal eye in the binocular brightness matches of a unilateraldeuteranope at various wavelengths.

Wavelength Logluminance for color-blind eyemgu luminance for normal eye

452 0.77475 0.77500 0.77538 1.01568 0.63591 0.45640 0.00681 0.00

a These values correspond to the intercept constants of Fig. 2.

for a brightness match between the two fields, and aand b are the slope and intercept constants, respectively.The slopes of all functions are identical and equal tounity; it is the intercept constant that serves to differ-entiate the several functions. This value is seen to varybetween zero for the reds, where brightness equality ofthe test fields corresponds to normal-eye luminance(and, indeed, physical) equality, and 1.0 for the green(538 mu), where the color-blind eye requires an addi-tional log unit of luminance in order to see the leftfield as equal in brightness to the right field. The valuesfor these intercept constants for the different spectralregions are listed in Table II. Since the relations on alogarithmic grid are as given, in general: Id= bln, whereb, the antilogarithm of the intercept constant, gives themultiple of the luminance for the normal eye thatmatches the luminance required by the color-blind eye.The value of b varies between 10.0 for green light and1.0 for red light.

DISCUSSION

For a report of equality of brightness of two binocu-larly viewed test fields, the luminance requirements forthe field seen by our subject's color-blind eye exceedthose for the field viewed by her normal eye in all butthe red spectral regions. This indicates a loss of lumi-nance for the dichromatic eye. The extent of thisluminance loss is a function of wavelength. It is greatestin the green and successively less and less in the blue-green and blue, yellow-green, and yellow; no detectableloss occurs in the red. Moreover, these selective spectralluminosity losses are maintained over the luminancerange tested.

Considerable evidence points to the fact that thebrightness of a stimulus depends not only on thefrequency of nerve impulses activated in each nervefiber but also on the total number of receptor-nervefiber units activated by the stimulus.6 Thus anyreduction in the apparent brightness of a foveallyviewed stimulus of constant luminance can be regarded

I S. Hecht, handbook of General and Experimental Psychology,edited by Murchison (Clark University Press, Worcester, Massa-chusetts, 1934).

as resulting from a decrease in receptors which normallyfunction in foveal vision. When such loss is demon-strated for particular spectral regions, the color receptortype maximally sensitive to radiant energy from thatspectral region may be presumed to be inactive, in astate of decreased activity or missing. It is in thismanner that the luminosity loss of protanopes has beenexplained.7 The reduced sensitivity at longer wave-lengths and the shortening of the curve in the red areinterpreted as indicating loss of those receptors whichnormally respond maximally to red stimulation.

The commonly held assumption that deuteranopessuffer no loss of any color receptor types,7 based on thereported similarity of the deuteranope's luminositycurve to that of the normal trichromat, predicts noselective spectral brightness losses for the deuteranope.Reference has already been made to the findings ofHecht and Hsia2 and the more recent ones by Hsia andGraham3 which do show reduced sensitivity values inthe luminosity functions of deuteranopes. Mention hasalso been made of the fact that Graham and Hsia4

determined the threshold luminosity curves of botheyes of our unilaterally color-blind subject. The selec-tive spectral luminosity losses found at threshold for thedeuteranopic eye of this subject are confirmed in thepresent study and are seen to be maintained at highphotopic luminance levels. There is good agreementbetween these two studies with respect to the order ofrelative losses incurred by various colors. In bothexperiments, green is most affected, with successivelydecreasing losses occurring in the following order:blue-green and blue, yellow-green and yellow; slightif any loss is sustained in the red.

Berger, Graham, and Hsia5 have measured thecritical fusion frequency functions at ten spectralregions for both eyes of this unilateral deuteranope.For all colors except red, the curve for the color-blindeye was displaced downward on the critical fusionfrequency axis with respect to the normal eye curve.At any given spectral region, the displacement wasapproximately constant over the luminance rangetested (3.5 log millilamberts). The magnitude of theshift was found to vary with wavelength. A com-parison of the brightness losses manifested in thepresent study with the subject's losses in critical fusionfrequency shows some discrepancies between the twowith respect to the order of relative losses at variousspectral regions. The loss in critical fusion frequencywas relatively greater in the yellow-green (568 mu) thanin the blue (475 and 452 m); yellow (591 m) andblue (452 m) showed approximately equal losses.These discrepancies may result from differential distri-bution of color receptor types within the retinal areasinvolved in the two studies, the critical fusion functions

7 F. H. G. Pitt, Proc. Roy. Soc. (London) 132, 101 (1944);G. L. Walls and R. W. Mathews, "New means of studying colorblindness and normal foveal color vision," University of CaliforniaPublications, Psychology 7, No. 1 (1952).

626 Vol. 48

September1958 VISUAL FUNCTIONS OF UNILATERALLY COLOR-BLIND. II 627

being measured for a 28-min test field while the binocularbrightness matches were made between fields subtending1.80 of visual angle.

The results of the present investigation are inharmony with our previously stated conclusion thatdeuteranopia probably represents a loss or inactivationof part of the receptor mechanism that normallymediates color vision, more specifically, the class ofcolor receptors having the maximum of its spectralsensitivity curve in the green portion of the spectrum.The binocular brightness match data show that selectivespectral luminosity loss for the deuteranope is not aphenomenon that exists only at cone threshold levels.At luminances as high as 2.5 log millilamberts, thecolor-blind eye requires ten times more green light thanthe normal for a judgment of brightness equality. Atsimilarly high photopic levels, the luminances for thenormal eye with blue and yellow lights must also bemultiplied by characteristic factors (5.89 for blue,2.82 for yellow) in order to match the luminancerequirements for the dichromatic eye. If the thresholdluminosity data were to be considered as indicatingnothing more than a selective elevation in the thresholdsof certain color receptors, then these sensitivity deficitsmight be expected to be overcome at the upper lumi-nance levels employed in the present study and thefunctions for green, blue, and yellow would be expected

to converge on the red data. Our results indicate thatsuch is not the case.

The hypothesis of a simple "collapse" or "trans-formation" system assumes that in deuteranopia thesensitivity of green receptors has become identical withor similar to that of red receptors due to a develop-mental failure in which the photosensitive substanceappropriate to green cones has not differentiated fromthe red photochemical substance; thus no receptor lossis claimed to be involved in deuteranopia. Indeed,Walls and Mathews,7 advocates of the collapse hy-pothesis, have stated that this notion is irreconcilablewith a brightness loss. They have also questioned thefindings of Hecht and Hsia2 on the grounds that conethreshold measurements cannot provide conclusiveevidence as to receptor loss and that unilateral deuter-anopes have not reported great reductions in brightness.The notion of a simple "collapse" or "transformation"system as responsible for the deuteranopic type ofdichromasy shown by our subject is not, in our opinion,supported by the implications of our findings, norcould our data be predicted or explained by an un-modified version of such a mechanism.

ACKNOWLEDGMENT

We wish to express our appreciation to our subjectfor the many hours of observation that she spent in thecourse of these experiments.