950 Proceedings of Royal Society of Edinburgh. [sess. 
of coloured objects. In the course of these I looked steadily 
for some time, in strong lamp-light, at a red table-cover and 
watched the gradual weakening of the colour, which ultimately 
became very grey. Quite suddenly the colour changed from 
greyish but distinct red to a fairly strong green, which could be 
looked at for some time, though a slight motion of the eyes would 
cause it to change back to red. The process could be readily 
repeated. The suddenness of the change was quite startling at 
first. I ascribed it, at the time, to an action analogous to mus- 
cular fatigue. If one pushes against a reverse force, which can 
just be overcome, the muscles ultimately become fatigued and 
resist less and less strongly and more and more spasmodically. A 
sudden uncontrollable diminution of effort at last enables the 
reverse force to produce reverse motion. 
About two years ago I made, quite unintentionally, an observa- 
tion which negatives the universality of the truth of the fourth 
statement. Being awake during the night, I was watching the 
coloured luminosity in the eye — presumably Dr Burch’s ‘dazzle- 
tint ’ — and noting its changes. The main colours are greenish 
and violet, and these appear to be nearly complementary, for they 
give rise to a whitish light when they seem to border on each 
other and presumably overlap. They sometimes change with 
great rapidity, suddenly spreading out from centres, suddenly 
breaking up in centres and rolling off throughout the field of 
vision. At other times they are more fixed. I could see them 
even against the dark sky, some of the colours being brighter and 
some darker than the background. As usual, faint stars were 
visible when regarded indirectly but disappeared when looked at 
directly. After a time, when the colours were more steady, I 
found that, by motion of the eyes, it was possible to get the 
patches removed temporarily from the central parts of the field ; 
and I then saw the faint stars well by direct vision. The usual 
want of power to see faint objects directly is very tantalising. 
The pleasure associated with the abnormal power of seeing them 
best by direct vision is quite indescribable. Continuous with- 
drawal of the line of vision from them produced continuous 
diminution of visibility ; and, whenever the line of sight was 
such as to bring the star to the edge of a dark patch of colour in 
