1888 .] Mr G. N. Stewart on Intermittent Light. 
445 
might easily be accomplished by working at night in a long corridor 
or even in the open air ; and by increasing the number of reflections 
from fixed mirrors placed at each end, so that the ray would have 
to travel several times backwards and forwards before it reached 
the eye, one might be able to get a stimulus consisting only of a 
thousand vibrations or even less. The behaviour of such exceedingly 
short flashes would be a subject of great interest, and calculated 
probably to help us to a knowledge of what retinal stimulation 
really is. For the present, however, I have been obliged to content 
myself with the minimum mentioned above. 
Result. — The result of my observations may be given in a single 
sentence. For the shortest stimuli I was able to use, there was no 
noticeable change in the intensity, once complete fusion had been 
reached — that is, no noticeable departure from Talbotts law. Even 
for the faintest light no definite variation appeared. If there be 
a minimum length of stimulus below which no summation takes 
place, it certainly lies below 
1 
s;Mo,ooo 
of a second for the weakest 
light. 
On certain Colour Phenomena caused by Intermittent Stimulation 
with White Light. 
In the course of the above investigation, I came upon a pheno- 
menon which, so far as I know, has not been previously recorded. 
When the mirror was turned slowly, but with gradually increasing 
speed, and a beam of white light reflected from it directly 
to the eye {i.e., without going to the fixed mirror), a series 
of colour changes was seen, which the first two or three times I 
noticed them I was inclined to attribute to an excessive stimula- 
tion of the retina. But, on investigating the occurrences a little 
closer, I found such a regularity under different circumstances, with 
different intensities of light, and with different light sources, that 
there could not be the smallest suspicion of anything accidental. 
As the appearances were seen when a blackened disk, with a hole 
cut in it near the circumference, was substituted for the mirror, they 
could not be due to any chance property of the latter. To eliminate 
any possible peculiarity of vision in my own eye, I asked several 
