September 9, 1898.] 



SCIENCE. 



311 



tion. Hering regards some of these as phe- 

 nomena of adaptation, and shows, in his 

 remarkable paper on Purkinje's phenome- 

 non, published in 1895, that the state of the 

 retina, as conditioned by previous exposure 

 to light, affects greatly the perception of 

 color. Attention to this fact is necessary in 

 photometric comparisons. The eye should 

 be kept in the same condition, as nearly as 

 possible, throughout any set of observations. 

 But careful measurements by von Kries and 

 others, keeping the eye carefully adapted 

 for brightness, have proved that Purkinje's 

 phenomenon exists, whatever the state of 

 the eye, though much modified by adapta- 

 tion. 



Another cause of false color-appreciation, 

 insisted upon by Hering, is the pigmenta- 

 tion of the macula. This is certainly of 

 importance. In experiments with color- 

 disks the apparatus, to secure consistent 

 results, must always be placed at the same 

 distance from the eye. A color-match made 

 with the disks close to the eyes will 

 in general not hold if the observer steps 

 back a few feet, because the macula covers 

 in the two cases a very different portion of 

 the retinal image of the disks. The region 

 corresponding to the macula, indeed, can 

 generally be seen projected upon the sur- 

 face of the revolving disks, as a spot in- 

 clining more to reddish than the remainder 

 of the surface. The intensity of the yellow 

 pigment, differing in the eyes of different 

 people, must affect their general perception 

 of color. 



The well-marked divisions of color-blind, 

 into green-blind and red-blind, as they 

 would be called in the Young-Helmholtz 

 theory, were explained by Hering as due to 

 the more or less deeply pigmented macula. 

 But the utter inadequacy of this explanation 

 has been abundantly shown. 



Perhaps the most striking difference be- 

 tween the Hering hypothesis and the facts 

 is shown in the distribution of color-sense 



in different parts of the retina. Ability to 

 distinguish colors decreases gradually from 

 the center to the exterior. The distinction 

 of red and green disappears first, then the 

 yellow becomes uncertain, and finally blue 

 disappears, the outer zone of the retina be- 

 ing devoid of color-sense. The zones are 

 not well defined, varying with the bright- 

 ness of the light and the size of the colored 

 surface. But making due allowance for 

 these circumstances, the area within which 

 red is distinguishable differs from that occu- 

 pied by green, and the yellow sensation 

 differs in extent from the blue. If red and 

 green, or yellow and blue, are due to the 

 presence of the same visual substance it 

 seems that the boundaries should be co-ex- 

 tensive. 



Even the sensation of white presents 

 similar variations. There are, as has been 

 already said, three cases in which the color- 

 sense is wanting: the totally color-blind 

 eye, the normal eye in faint light, and the 

 periphery of the retina. The brilliant dis- 

 covery of Hering, in 1891, that the distribu- 

 tion of brightness in the spectrum in the 

 first two cases is the same, aroused great 

 interest in the theory of the sensation of 

 white, and went far toward establishing its 

 position as a distinct and separate sensation. 

 The third case, it was taken for granted, 

 fell under the same law. But in 1896 von 

 Kries showed that the distribution of bright- 

 ness in the spectrum as seen by the outer 

 zone of the retina is different, being practi- 

 cally the same as in the central portion, 

 with its maximum in the yellow, and that 

 the peripheral zone in the retina of a color- 

 blind person shows the same deficient sen- 

 sation for the longer wave-lengths as in the 

 color-perceiving portion of the eye. 



This is a matter of so much interest that 

 I have re-examined it with the fiicker pho- 

 tometer, with results differing materially 

 from those of von Kries. According to my 

 experiments, the brightness of the colors of 



