RECENT PROGRESS IN OPTICS. 133 



area on a gray background appears surrounded by its complementary 

 tint is familiar enough. For its explanation it bas been common to 

 assume that there is unconscious motion of the observer's eyes, incip- 

 ient retinal fatigue, an error of judgment, or fluctuation of judgment. 

 This has been tested by A. M. Mayer (American Journal of Science, 

 July, 1893), who ingeniously devised methods for showing these con- 

 trast phenomena on surfaces large enough to match the colors witli 

 those of rotating color disks, and thus to arrive at quantitative state- 

 ments of their hues. When viewed through a small opening in a 

 revolving disk, the subjective contrast color was unmistakably percep- 

 tible when the duration of passage of the opening was less than one- 

 thousandth of a second. The same effect was obtained in a dark room 

 with instantaneous illumination of the colored surface by the strong 

 spark of an electric influence machine. The duration of illumination is 

 thus almost infinitesimal, certainly not more than one ten-millionth of a 

 second. The hypothesis of fluctuation of judgment is thus shown to be 

 wholly untenable. I have performed most of these experiments, either 

 with Professor Mayer or separately, and my testimony can therefore 

 be united with his. The case is quite analogous to that of the percep- 

 tion of binocular relief, which was once explained as the product of a 

 judgment, but was found to be always possible with instantaneous 

 illumination. Professor Mayer has devised a disk photometer based 

 on color contrast, with which the error of a single reading was found 

 much less than with the Bun sen photometer. 



The rotating color disk has been applied by O. N. Eood (American 

 Journal of Science, September, 1893) to the determination of luminosity 

 independently of color, by taking advantage of the flickering appear- 

 ance on a rotating disk upon which two parts have different reflecting 

 powers. An extreme case of this is that of a white sector upon a black 

 disk. At a certain critical speed the retinal shock due to momentary 

 impression by white light becomes analyzed into the subjective impres- 

 sion of spectral colors, the duration of the retinal sensation varying 

 with the wave length of the incident light. The law of this variation 

 has been studied by Plateau ("Dissertation sur quelques proprietes des 

 impressions produits par la lumiere sur l'organe de la vue," Liege, 1829), 

 Nichols (American Journal of Science, October, 1884), and more 

 recently with much precision by Ferry (ibid., September, 1892), who 

 showed that retinal persistence varies inversely as the logarithm of the 

 luminosity. For a given source of light separated into its spectral 

 components, the yellow is the brightest. For this hue accordingly the 

 retinal impression is shortest, and for violet it is longest. 



Under appropriate conditions the after-effect on the retina has a cer- 

 tain pulsatory character, as first noted by C. A. Young (Philosophical 

 Magazine, Yol. XLIII, p. 343, 1S72) in 1872, and carefully studied 

 within the last few years by Charpentier ("Oscillations retiniennes," 

 Comptes rendus, Yol. CXIII, p. 147, 1891) in France, and Shelford 



