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£.. L. Nichols—Color Impressions upon the Retina. 251 
of time the retina has been exposed. Plateau, in the paper 
already cited, gives a curious law for the velocities necessary to 
produce apparent uniformity of surface upon black and white 
disks. He found that any two disks composed of alternate 
black and white sectors will attain a uniform tint at the same 
velocity, provided the black sectors in the one are equal in 
width to each other and to the white sectors in the other disk. 
That is to say, a black disk with narrow white sectors presents 
a homogeneous surface at the same velocity as does a white disk 
with the same number of similiar narrow black sectors; and 
this holds true, no matter what the number and width of the 
sectors may be. In a more recent paper, Plateau gives some 
measurements of the rate of rotation of such disks. e law 
was not put to a test in his experiments, as it might have been, 
since the disks for which measurements were made were black, 
with white sectors never exceeding the black ones in width. 
he present writer, to determine the manner in which the 
duration of the retinal image varies with the length of exposure, 
Exposure of retina, Duration of the image, 
0°0124 seconds. 0°0954 seconds. 
0°0274 « , 0°0824. ** 
0.0717 * OW? 3 
0°1314 + 0°0654 . 
0-2316 - 0°0463 = ** 
0°4506 “ 0°0409 “ 
0°7566 éc 0°0327 “ 
These values are not in accordance with Plateau’s law. The 
aoe unfortunately incomplete, indicates that upon Increasing 
e time i i i j 
infinitesimal exposure, and to become zero for an exposure ~ 
i pare to a single revolution of the disk. We know by the study — 
Of after-images that their duration after short exposures of the © 
