W. LeConte Stevens—fecent Progress in Optics. 385 
there is no direct proof of the existence of the assumed gray 
and color molecules, it may be answered that Helmholtz him- 
self fully recognized the uncertainty of the assumption that 
three different sets of nerves respond to the three fundamental 
eolor-sensations, and he admitted that these may be only 
different activities in the same retinal cone. The supposition 
of three adjacent cones, responding respectively to the three 
fundamental sensations, is made only for the sake of greater 
convenience in discussion. 
Indeed there is still much for us to learn regarding the 
nature of color-sensation. Among the yet unexplained phe- 
nomena are those of simultaneous color-contrast. The fact 
that a small brightly-colored area on a gray background appears 
surrounded by its complementary tint is familiar enough. For 
its explanation it has been common to assume that there is 
unconscious motion of the observer’s eyes, incipient retinal 
fatigue, an error of judgment, or fluctuation of judgment. 
‘This has been tested by A. M. Mayer,* who ingeniously devised 
methods for showing these contrast phenomena on surfaces 
large enough to match the colors with those of rotating color 
disks, and thus to arrive at quantitative statements of their 
hues. When viewed through a small opening in a revolving 
_ disk the subjective contrast color was unmistakably perceptible 
when the duration of passage of the opening was less than 
spoath of asecond. The same effect was obtained in a dark 
room with instantaneous illumination of the colored surface 
by the spark of an electric influence machine. The dura- 
tion of illumination is thus almost infinitesimal, certainly 
not more than z5gy/o79 Of a second. The hypothesis of fluc- 
tuation of judgment is thus shown to be wholly untenable. I 
have performed most of these experiments, either with Prof. 
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. Prof. 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 Bunsen 
photometer. | | 
The rotating color disk has been applied by O. N. Roodt to 
the determination of luminosity independently of color by 
taking advantage of the flickering appearance 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 
* This Journal, July, 1893. + Ibid., September, 1893. 
