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of Edinburgh , Session 1872 - 73 . 
choroid ; anti Mosier compared it to the action of light on a sen- 
sitive photographic plate. 
It is evident that, in accordance with the principle of the trans- 
ference of energy now universally accepted, the action of light on 
the retina must produce an equivalent result, which may be 
expressed, for example, as heat, chemical action, or electro-motive 
power. It is well known that the electro-motive force of a piece 
of muscle is diminished when it is caused to contract by its normal 
stimulus, the nervous energy conveyed along the nerve supplying 
it ; and similarly a nerve suffers a diminution of its normal electro- 
motive force during action. In the same manner, the amount and 
variations of the electro-motive power of the optic nerve, affected 
secondarily by the action of light on the retina, are physical ex- 
pressions of certain changes produced in the latter ; or, in other 
words, are functions of the external exciting energy, which in this 
case is light. Considerations such as these led us to form the 
opinion that the problem of what effect, if any, the action of light 
has on the electro-motive force of the retina and optic nerve, 
would require for its investigation very careful and refined experi- 
ment. 
The inquiry divided itself into two parts, — first, to ascertain the 
electro-motive force of the retina and nerve; and, second, to observe 
whether this was altered in amount by the action of light. The 
electro-motive force of any living tissue can be readily determined 
by the method of Du Bois-Revmond, This great physiologist 
found that every point of the external surface of the eyeball of a 
large tench was positive to the artificial transverse section of the 
optic nerve, but negative to the longitudinal section. This he 
accomplished by the use of his well-known non-polarisable 
electrodes, formed of troughs of zinc carefully amalgamated, con- 
taining a solution of neutral sulphate of zinc, and having cushions 
of Swedish filter paper on which to rest the preparation. To pro- 
tect the preparation from the irritant action of the sulphate of 
zinc, a thin film or guard of sculptor’s clay, moistened with a '75 per 
cent, solution of common salt, and worked out to a point, is placed 
on each cushion. These electrodes were connected with a galvano- 
meter, and the preparation was placed so that the eyeball, carefully 
freed from muscle, rested on the one clay-guard, while the transverse 
