PHYSIOLOGICAL ACTION OF LIGHT. 163 
(4.) It was found in this eye that at a distance of one foot, on the impact 
of light, there was a depression of 90° to 103°; at a distance of ten feet, there 
was a depression of from 15° to 20°. Here the depression did not at all cor- 
respond to the intensity. At a distance of ten feet, when the amount of light 
was ;oth part of what it was at one foot, the diminution in the alteration of 
electro-motive force by the action of light was not z$pth, but ith of the total 
amount. 
3. The Swimming Crab; and, 4, the Spider Crab.—These animals, dredged 
from a depth of twenty fathoms from the Firth of Forth, had eyes which were 
found sensitive to light. 
5. The Hermit Crab.—The length of the eye examined in this case was 
about one millimetre. Its breadth was about one-fourth of a millimetre. Still 
it was found very sensitive to light. 
XV.—EXPERIMENTS ON THE EFFECT OF FATIGUE ON THE EYE. 
A few experiments, preliminary to a more detailed examination of the ques- 
tion, have been made as to the effect of fatigue on the eye, as estimated quanti- 
tatively by the amount of change in the electrical potential of the visual struc- 
tures. We may, in this communication, shortly allude to these. Suppose we 
place the eye of a frog in connection with the clay pads of the troughs of the gal- 
vanometer. The box with the draw-shutter, already described, is placed over it, 
and one candle, giving a steady flame, is put a distance of twelve to sixteen inches 
from the box. The draw-shutter is now opened, and the amount of deflection 
on impact of, during the continuance of, and on the removal of, light is noted. 
If we then light another and exactly similar candle, so as to double the amount 
of light, the variation in the electro-motive condition of the eye is not much 
altered, certainly not to the extent of double the amount of that produced by 
one candle. If we then extinguish one candle, a slight alteration occurs ; but 
when we extinguish the remaining candle, there is at once a great variation in 
the deflection of the galvanometer. That is to say, the eye becomes less sensi- 
tive to any increase in the intensity of light after it has become for some time 
subjected to the action of a light of certain intensity ; or, in other words, it be- 
comes fatigued. This experiment also shows that the eye is more sensitive to 
variations in light of weak intensity than to variations in light of great intensity. 
XVI—MobpEs oF GRAPHIC REGISTRATION IN THESE EXPERIMENTS. 
During the course of our observations we have found it necessary to construct 
a true graphical representation of the variations of the electro-motive force occa- 
sioned by the impact, duration, and removal of light. It is clear that to register 
minute galvanometrical alterations from a THomson’s instrument, the only plan 
