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The visual mechanism of a normal observer is so constructed that objects keep nearly their daylight colors even when the illuminant departs markedly from average daylight. The processes by m eans of which t he observer becomes adapted to the illuminant or discounts most of the effect of a nondaylight illuminant are complicated; they are known to be partly retinal and partly cortical. By taking into account the various fragments of both qualitative and quantitative information to be found in the literature, relations have been formulated by means of which it is possible to compute approximately the hue, saturation , and lightness (tint, value) of a surface color from the tristimulus specifi cations of the light reflected from the surface and of the light reflected from the background against which it is viewed. Prelimin ary observations of 15 surfaces under each of 5 different illumina nts have demonstrated the adequacy of the formulation, and have led to an approximate evaluation of the constants appearing in it. More detailed and extensive observations have been carried out in the psychological laboratories of Bryn Mawr College, and these observations have resulted in an improved formulation.

The reflection factor of a plane boundary between two media has been computed by the Fresnel formula for unpolarized, perfectly diffused incident light as a function of the relative index of refraction of the media. Because of total internal reflection, the factors depend importantly upon wbether the diffuse flux is incident externally or internally. For example, diffuse light in air incident on the plane surface of glass of index 1.5 is 9.2 percent reflected; but if the perfectly diffuse light is incident internally, the reflection factor is 60 percent.In the optical specification of light-scattering materials 1 by means of the absorption and scattering coefficients of the Kubelka-Munk theorY,2 it is frequently necessary to consider the effect of both ext ernal and internal reflection of light diffusely incident on a plane boundary between two media of different indices of refraction. 3 The refl ectance at such a boundary has therefore been computed by application of the well-known Fresnel formula. The results are shown in table 1 as a function of the ratio, m, of the index of refraction of the denser medium to that of the rarer. The reflectance for perpendicular incidence, r, computed from the formula (1) is also shown in the table.The reflectance for completely diffused unpolarized light incident externally was computed from the approximate formula given by McNicholas: 4 ... ,. where r", is the reflectance by the Fresnel formula for unpolarized light incident on the surface from the rarer medium at an angle, q" from the perpendIcular to the surface. The angular interval, t:.q" used in these summations was 0.04 radian.I Dea ne B. Judd (with the collaboration of W.

It is w(l ll established that about 2 percent of otherwise normal human males are confu~e rs of red and green from birth. There is considerab le in terest in the question: What do red-green confusers see? From a kn owledge of the normal color perceptions corresponding: to deuteranopic and protanopic red and green, we may not only understand better why colo r-blindness t ests so metimes fai l, and so be in a position to develop improved tests, but a lso t he color-defi cient obser ver may understand better the natu re of hi s color-confusion and he aided to avo id the ir consequences. If an obse r ve r has trich romatic vision over a portion of his total retinal a rea, and dichromatic vision ove r anothe r portion, he may give valid testimon y regard ing the color perceptions characteristic of the particu lar form of dichromatic visio n possessed by hi m. Preeminen t among s uch obse r vers are those born with o ne no rmal eye a nd one dich ro matic eye. A review of the rather consid erable li te rature on this s ubject shows t ha t the color perceptions of both protanopic and deute ranopic obse r vers are con fin ed to two hues, ye ll ow and bl ue, close ly li ke those perce ived under us ual conditions in the s pectrum at 575 and 470 mIL, r cspectively, by normal obser ve rs. By combining this re ult wit h standard res ponse fun ctions recently derived (Bureau Resea rch Pape r RP 1618) fo r p rotanopic and deute ranopic vi ion , it has been possible to give quantitative estimates of the co lor pe rceptions typical of these ob. e rve rs fo r the whole range of colo rs in t he :l\1unse ll Book of Co lor. These estim ates take t he form of p rotanop ic and deute ranopic Munse H notations, and by using them it is possible not o nl y to arrange the l\lunseli papers in ways that p res umably appear orderl y to red-gree n confusing d ichromats, b u t a lso to get immediately from t he no tation s an accurate id.ea of the colors us ua ll y perceived in these a rran gements by deute ranopes and protanopes, much as the ord in ary l\lunse ll notations ser ve to describe t he vi sual co lor pe rceptions of a normal obse r ver.