PROPERTIES OF THE RETINA. 349 



brilliant spectrum the maximum brightness is in the yellow, but 

 with a feeble illumination it shifts to the green. This fact accords 

 with what is known as the " Purkinje phenomenon/' namely, the 

 changing luminosity and color value of colors in dim lights. As the 

 light becomes more feeble the colors toward the red end of the 

 spectrum lose their quality, the blue colors being perceived last of 

 all, just as in late twilight it may be noticed that the sky remains 

 distinctly blue after the colors of the landscape become indistin- 

 guishable. It should be added that the " Purkinje phenomenon " 

 is true only for the parts of the retina lying outside the fovea, 

 that is, for the peripheral field. As the light grows dimmer the 

 perception of blue is lost first in the fovea, so that with a certain 

 feebleness of illumination the central field becomes blue-blind. 

 With a very feeble illumination the dark-adapted eye becomes 

 practically totally color blind. 



Qualities of Visual Sensations. The different qualities of our 

 color sensations may be arranged in two series: an achromatic 

 series, consisting of white and black and the intermediate grays, 

 and a chromatic series, comprising the various spectral colors, 

 together with the purples made by combination of the two ends 

 of the spectrum, red and blue, and the colors obtained by fusion of 

 the spectral colors with white or with black, such, for instance, as 

 the olives and browns. 



The Achromatic Series. Our standard white sensation is that 

 caused by sunlight. Objects reflecting to our eye all the visible 

 rays of the sunlight give us a white sensation. This sensation, 

 therefore, is due primarily to the combined action of all the visible 

 rays of the spectrum, each of which, taken separately, would give 

 us a color sensation. White or gray may be produced also by the 

 combined action of certain pairs of colors, complementary colors, 

 as is described below. Black, on the contrary, is the sensation 

 caused by withdrawal of light. It must be emphasized that in 

 order to see black a retina must be present. It is probable that 

 a person with both eyes enucleated has no sensation of darkness. 

 That black is a sensation referable to a condition of the retina is 

 made probable also by the interesting observations recorded by 

 Gotch,* namely, that when an eye that has been exposed to light 

 is suddenly cut off from the light there is an electrical change in the 

 retina, a dark response, similar to that caused by throwing light on 

 a retina previously kept in the dark. Blackness, therefore, is a 

 sensation produced by withdrawing light from the retina, and a 

 black object is one that reflects no light to the eye. Black may be 

 combined with white to produce the series of grays, and when com- 

 bined with the spectral colors it gives a series of modified color tones, 



* Gotch, " Journal of Physiology," 29, 388, 1903. 



