102 
Proceedings of the Royal Society 
section of the optic nerve was in contact with the other. By 
following Du Bois-Reymond’s method, we have had no difficulty 
in obtaining a strong deflection from the eyes of various rabbits, a 
cat, a dog, a pigeon, a tortoise, numerous frogs, and a gold-fish. 
The deflection was frequently so much as to drive the spot of light 
off the galvanometer scale. 
With regard to the second question, namely, whether, and to 
what extent, the electro-motive force would be affected by light, 
we found more difficulty. The method followed was to place the 
eyeball on the cushions in the manner above described, to note the 
deflection of the galvanometer needle, and then to observe whether 
or not any effect was produced on the impact of a beam of light, 
during its continuance, and on its removal. In a few of our earlier 
experiments, we used Du Bois-Reymond’s multiplying galvano- 
meter; but finding the amount of deflection obtained was so small 
that the effect of light could not be readily observed, we have 
latterly used Sir W. Thomson’s exceedingly sensitive reflecting 
galvanometer, kindly lent us by Professor Tait. We met also with 
secondary difficulties, such as the dying of the nerve, the impos- 
sibility of maintaining an absolutely constant zero and an absolutely 
constant amount of polarity, the effects of heat, &c. ; but these 
difficulties we have overcome as far as possible by the most approved 
methods. The changes in polarity of the apparatus occurred 
slowly, and could not be mistaken for the changes produced by the 
action of light, which we found occurred suddenly, and lasted a 
short period of time. It is also important to state, that the de- 
flections we observed do not at present profess to be absolute, but 
only relative values. 
About 500 observations were made previous to the date of this 
first communication, and we took every precaution to obtain 
accurate results. The effects of heat were carefully avoided by 
covering over the troughs on which the eye under examination 
rested, with a spherical double shell of glass, having at least an 
inch of water between the walls. 
The results we have arrived at are as follow : — 
1. The action of light on the retina is to alter the amount of the 
electro-motive force to the extent of from three to seven per cent, 
of the total amount of the natural current. 
