842 Proceedings of Royal Society of Edinburgh. [sess. 
twenty-four hours in absolute darkness. We kept the animals in 
darkness for a much longer time — in some cases for three or four 
days. We also used a specially constructed trough, by which the 
animals were subjected to red, green, or blue light for long periods 
by the use of appropriate filters. It was not difficult, as could be 
proved by a spectroscopic examination of the fdtering medium, to 
obtain fairly pure red and green light, but we cannot say the same 
of blue. The retinas of the animals could thus be, as it were, 
“ adapted” to red, green, or blue rays. As Professor Gotch is 
working at this subject, we do not propose to go further, in the 
meantime, with these experiments, but we recognize their great 
importance as being an objective method of investigating the 
validity of the various theories of colour vision. With eyes so 
“adapted” we obtained electromotive effects considerably greater 
in magnitude than those noted by Gotch. We also made some 
experiments with Wood’s ultra-violet light filter, and with “light 
off” we detected a slight upward movement of the light on the 
galvanometer screen. As, with a sensitive eye, the smallest 
leakage, or the slightest movement of the box in performing the 
experiment, might account for this, we agree with Gotch that no 
proof has yet been given that the ultra-violet rays produce any 
effect. 
( Issued separately August 29 , 1905 .) 
