244 EF. L. Nichols—Color Impressions upon the Retina. 
retinal image varies with the wave-length of the impinging ray 
remains in the condition in which Plateau left it. The results 
of his measurements are given in the following table: 
Duration of color-impressions according to Plateau. 
Color of the Total i nore of oe vubaip of 
alternate sectors. the the 
hite, 0°35 seconds, 0° 00796 ‘seconds, : 
Yellow, 0°35 0°00798 : 
io ee 0°34 a 0°00966 = 
Blue, O32. g°01229  “ 
The total duration of the image was determined from the 
slowest rotation which would sairely blur the black sectors of: =a 
the revolving ak. The time during which the image re 
mained undimmed was paleu lated from rigs velocity necessary 
to give the face of the disk a uniform tin 
Emdmann found the duration of the i seloss to is 
White =0-25" Red= 
Yellow=0°27’ Hao na to 0 0-29" 
These values probably refer to the total duration. Helm- 
holtz* found the undimmed duration of the image when the ~ 
disk was illuminated by lamp-light to be a, of a second, by 
moon-light 315 of a second. a 
oe. "this phenomenon worthy of further study, the 4 
writer has determined, by a modification of the method of 
Plateau, the persistence of the retinal image asa iacdae of | 
the wave-length of the ray producing it. The measure of the 
duration of the image, adopted in the experiments to be de 
scribed, was the longest ‘ceeeval which could be allowed to 
occur between successive exposures of the retina without intet-".> = 
fering with apparent ssijotathcd of vision. An sedge a one- 
prism spectroscopet was so placed as to give a fairly pure spec 
trum of a beam of diffuse white daylight; the Hreunhoter's 
lines being well defined. A black disk 240™ in diameter, with 
four narrow open sectors 5™" each in width, was rotated before 
eo oe th vt iy olin noptipes Optik, p. 3 
+ Professor Roo rn Chromatics,” a treatise e ob ch the writer owes 
much, se propose me the a eis the pho oscope and revolving disk for the study : 
the du jag of retinal images. He says (p. 206), “Dr. Wolcott Gibbs sug- 
to the author a method which would probably solve this problem in & 
Sstiatadhory manner, and which is about as follows: With the aid of a spectro- 
w 
flicker and presented a steady uniform appearance. This observation would give 
correctly the eral ‘Sine which the impression remained with diminished : 
strength on age eye, in the case of the selected color.’ 
