1888. ] and on a Photo- Voltaic Theory of Vision. 311 
My best thanks are due to Prof. J. J. Thomson and Mr Glaze- 
brook for much kind assistance and advice, and also to my friends 
Mr Jones and Mr Masom, who were good enough to place their 
eyes at my disposal for the experiment on fluorescence. 
Fig. 4. 
II. On a Photo- Voltaic Theory of Vision. 
Prof. Dewar and Dr M’Kendrick have shown’ that there is a 
considerable E.M.F. between the anterior portions of the eye 
and a transverse section of the optic nerve. Making contact by 
means of unpolarisable electrodes, a current was led round the 
coils of a galvanometer. It was found that this current was 
altered in amount when light fell on the retina, but not when 
light fell on other parts of the eye, such as the optic nerve. 
It has further been shown by numerous experimenters, in- 
eluding Ritter, Helmholtz and Du Bois-Reymond, that a current 
in the optic nerve produces the sensation of light. The question 
then is, by what means does light falling on the retina produce or 
modify the current in the optic nerve? On comparing the laws 
of perception of light with the photo-voltaic properties of selenium 
as determined by Prof. W. G. Adams and Mr Day’, there are 
found to be some striking resemblances, though of course 
selenium can hardly be the substance to which the retina owes 
its sensitiveness. It may be as well here to enumerate some 
of the properties of selenium, and then to point out their analogy 
to the laws of light-perception. 
(1) When a current flows in selenium it produces a two- 
fold effect ; (a) an increase of resistance in the direction of the 
current and a decrease in the opposite direction, (b) an opposing 
E,M. F. due to some kind of polarization. Both these effects 
persist for an appreciable time after the current has ceased. 
(2) When light falls -on selenium it causes (a) a decrease of 
resistance which persists for a short time after the light is shut 
off; (b) an E.M.F. which shows neither lag nor persistence 
in any appreciable degree; (c) fatigue, which causes this E. M. F. 
to fall off rapidly as the action of light is continued, and which 
is In some measure selective: the selenium showing most fatigue 
for the colour to which it has been exposed. 
(3) The rays which affect the electrical properties of scle- 
nium are precisely the visible rays; the ultra-red and ultra- 
1 Edin. Phil. Trans. xxvit. 141. 
2/Prac. A. )S'. SXMla ao exkoyel Gon xy. 193; 
