E. L. Nichols—Color Impressions upon the Retina. 247 
Series C, 
[Spectrum brighter than in Series B.] 
Revolutions nterval between Duration of 
Spectral region. per minute. exposures of retina. r 
7420 (red 190 0°0769 seconds. 0°00209 seconds. 
6463 (orange) 228 00641 =“ 00176). « Ff 
» 6025 (yellow) 280 0°0523 “: 0°00144.— ** 
5415 (green) 212 0°0690 0°00188 * 
4784 (blue 170 0:0860 0°00234 2 
4382 (violet) 136 0°1072 - 0°00286 s 
Inspection of these three tables and of the accompanying 
curves shows that the image formed by yellow light is less per- 
sistent than that produced by any other color, and that the 
aA. 
16" 
“14"" 
"10" 
' duration of the image increases rapidly as we approach either 
end of the spectrum. Aside from the inaccuracies, inevitable © 
Tegion in each curve. It will be seen that the duration of the 
image is least in the series corresponding to the brightest spec- 
trum and greatest with the darkest spectrum; and that changes 
In brightness affect the persistence of the images formed by the 
different spectral regions in nearly the same proportion. 
: The law of the dependence of the duration of color-impressions 
_ Upon the intensity of the ray could doubtless be determined by 
