404 PROCEEDINGS OF THE AMERICAN ACADEMY 



pearances be the true one, and if my observations be verified, it would 

 seem that the hypothesis of throe sets of cones must be abandoned.* 



If, then, we do really reach the ultimate possible limits of surface- 

 perception without approaching one step towards the analysis of white 

 light into its elementary colors, the only remaining hypothesis seems to 

 be that they are distinguished in different planes of the retina. This, 

 indeed, is countenanced by the calculation of Helmholtz that, when 

 the eye is accommodated for a white object at convenient distances, the 

 focus of the violet rays is .434 mm., or twice the thickness of all the 

 layers of the retina in front of the focus of the red *rays. Indeed, on 

 the undulatory theory, the difference of wave-lengths must itself be a 

 function of perception, which cannot therefore take place in a mathe- 

 matical plane. Analogies of light and sound at once suggested to me 

 sympathetic vibrations of the minute segments of the retinal cones, 

 whose diameters, according to Max Schultze's measurements, are about 

 the same as those of the wave-lengths near the red end of the spectrum. 

 Accordingly, the following series of observations on positive, or inci- 

 dental after-images, which have never been very fully investigated, was 

 made : — 



A series of bright-colored pieces of paper, four inches square, were 

 fastened to a long strip of pasteboard, in the order in which they occur 

 in the spectrum : a movable slit, of somewhat less diameter than the 

 squares of paper, allowed any color to be seen by itself, without the 

 effects of contrast. Positive after-images of each of these colors were 

 formed, first by opening the eyes as suddenly and closing them again 

 as quickly as possible, about once in a second, eight or ten times, — an 

 experiment which was afterwards varied by illuminating the squares in 

 a dark room by an electric spark, and later by observing, in the same 

 manner, a solar spectrum, cast by two prisms of rock salt. 



Beginning at one end of the spectrum, and trying each color succes- 

 sively, it was observed that, near the middle of the spectrum, the first 

 phase of the positive after-image is nearly or quite white. 



To my own eye, this effect is somewhat greater with highly saturated 

 blue than with green, which appears dazzling white even beside white 



* Of course, the purest colors obtainable in silk contain the whole spectrum, 

 and the only precaution adopted was that of tiring the eye for the complement- 

 ary color before fixating the fibres. Of ten High School boys, who were induced 

 to try the experiment for several consecutive days, and, without being told what 

 was expected, to draw the lines as they appeared, three represented them as 

 wavy, and could observe no difference in the size of the curves in the white and 

 in the colored fibres. 



