

1905.|] — On Colour- Vision by Very Weak Light. 209 
Between 25 and 60 minutes I ascertained that striking the forehead smartly 
with the hand when the bands were just bright enough to show colour caused 
them to appear colourless for a few seconds, but on repeating the experiment, 
at 105 minutes this effect was no longer produced. 
This result is, I consider, of great interest. For if a pair of differently 
coloured bands, considerably brighter than the minimum visible, appear for a 
few .moments colourless when the after-effects of previous illumination, or 
dazzle- tints, receive a transient intensification, the inference is plain that the 
colours of the minimum visible must be far more liable to be masked even. 
without such intensification, and that no observation as to the impossibility’ 
of distinguishing colours in the minimum visible can be valid unless it has 
been ascertained that the dazzle-tints have completely subsided. 
Relistion of the Violet-blue : Red Rati for the Mimmum Visible by Daylight, to. 
that for the Minimum Visible after Two Hours in Total Darkness. 
_ In making these measurements I grasped the eye-piece of the spectroscope’ 
with my left hand so as to exclude all extraneous light from the eye. The 
sensitiveness of the eye changes so rapidly that it is necessary to have some 
time limit within which the bands must be perceived. The normal limit here 
adopted was oné minute. If they were glimpsed before 55 seconds or after 
65 seconds, the experiment was rejected. : 
- Experiment 9.—Sitting with my back to the window on a dull December 
ae with all blinds down but one, the violet-blue : red ratio was 70. 
After looking out of the window for a minute or so it was 341, and on 
looking at the sky, which was very dull and cloudy for 10 seconds, it rose to 
418. Ten minutes’ rest with my back to the window brought it to 96. After 
writing for about 5 minutes on white paper it was 185. Ten minutes spent 
talking in an upstairs room brought it to 364, and after another 10 minutes 
with my back to the light it was 111. 
— Experiment 10.—On another occasion I took a series of measurements at 
half-hour intervals, working in another room in the intervals. The first 
measurement was taken directly after setting up the apparatus, and the 
second after going about the laboratory collecting materials for my other 
work. In the former case the influence of the subdued light of the room in 
the basement in which the apparatus was situated is evidenced by the lower 
reading and in the latter the effect of the more brightly illuminated roons is 
very marked. The last three, when the intervals were spent under fairly 
constant conditions, agree. tolerably well. 
- Hach measurement was made independently, the Nicol being fae back 
