
1905. ] On Colour-Vision by Very Weak Lnght. 207 
to 7-6 times its previous amount, to match the blue-violet. With a weaker 
light 11:27 times the amount of red was required, and after 45 minutes, with 
a still weaker light, 16°82 times the original proportion of red was required to 
match the blue-violet. ; : 
This explains, I think, why the yellow of the spectrum shifts so much 
towards the red end with very weak light after a prolonged stay in darkness. 
Experiment 7.—In order to compare measurements made with light of equal 
intensities during the progress of the change, and after the eye had reached 
the constant condition, I arranged to remain continuously in the dark room 
while an assistant came at stated intervals and moved the lamp from one to 
another of a series of increasing distances from the paper screen. Each time 
I marked the position of the prism for which the red and the blue-violet 
appeared equally bright, and the final distance was the greatest at which I 
could distinguish the two bands. 
At the end of 45 minutes it was found necessary to bring the experiment 
to a conclusion before reaching the final stage of adaptation. The assistant, 
therefore, moved the lamp to each of the fixed distances in the reverse order, 
and I took a record, as quickly as possible, of the setting of the Nicol for 
which the colours appeared equal in brightness. The width of the slit 
remained the same throughout, having been adjusted at the commencement 
of the experiment so that the bands were, to the non-rested eye, of minimum 
visible luminosity when equal in brightness. 
In the table “Time” signifies the number of minutes after the room was 
darkened. “Intensity” represents the physical intensity of the band in 
During the Process of Resting in Darkness. 
Intensity of | Intensity of 


oe red. blue-violet. oe 
ee en Dene eee aan st | 
0. al ah 103° 28 31°83 Minimum visible. 
5 4°34 23°49 5°41 Easily visible. 
20 1-20 5°80 4°84 After blow on the eye. 
30 1°14 2°19 i203 Easily visible. 
45 1-00 1-00 1:00 Minimum visible. 
After Resting in Darkness. 
45 1:00 1:00 1°00 
46 1°34 2°06 1°53 
47-30 1-98 5-30 2°67 
49-30 4°97 23-08 4°64 
51 11°55 97°67 8°46 
