PHYSIOLOGICAL ACTION OF LIGHT. 1435 
to ascertain changes in the electro-motive force of the nerve. It has been 
shown by numerous physiologists that, in the case of a motor nerve, the electro- 
motive force suffers a negative variation ; that is, its amount is diminished during 
an active condition of the nerve. This negative variation of the natural electro- 
motive force may be looked upon as the electrical expression of the physiologi- 
cal process taking place in the nerve during the passage of the normal nerve 
current. Analogy would lead one to expect a similar change in the electro-motive 
force of a sensory nerve during the transmission along its fibres of the influence 
produced by the action of the normal stimulus on the terminal organ with which 
the sensory nerve is connected. Thus, variations in the electro-motive force of 
the retina and optic nerve may be regarded as functions of the external exciting 
energy, which in this case is light. 
Such considerations led to this investigation. It resolved itself into a mat- 
ter of careful experiment in two directions: 1sé, to determine the electro-motive 
force of the visual apparatus and optic nerve; and, 2d, to ascertain whether or 
not, and to what extent, this electro-motive force was affected by light. 
The only experiment bearing on the first of these questions, namely, the 
electro-motive force of the optic nerve, was one made by Du Bors-Reymonp, and 
thus described :* “ Having prepared the optic nerve of a large tench in such a 
manner that one extremity was the artificial transverse section, and the other 
the globe of the eye freed from all adherent particles of muscle, found that every 
point of the external surface of the ocular globe was positive to the artificial 
transverse section of the optic nerve, but negative to the longitudinal section.” 
From this experiment it therefore seemed to be quite possible, by using a deli- 
cate instrument, to obtain a measurement of the electro-motive force of the 
retina and optic nerve. 
With regard to the second question, namely, the action of light on this 
electro-motive force, the problem seemed difficult to solve, but an experiment 
made by Mr, now Justice, Grove, held out the hope of success. It is detailed 
as follows :t—‘“ A prepared daguerreotype plate is enclosed ina box filled with 
water, having a glass front, with a shutter over it. Between this glass and the 
plate is a gridiron of silver wire ; the plate is connected with one extremity of a 
galvanometer coil, and the gridiron of wire with one extremity of a Breguet’s helix 
—an elegant instrument, formed by a coil of two metals, the unequal expansion 
of which indicates slight changes in temperature ; the other extremities of the 
galvanometer and helix are connected by a wire, and the needles brought to 
zero. As soon as a beam of either daylight or the oxy-hydrogen light is, by 
raising the shutter, permitted to impinge upon the plate, the needles are deflected. 
Thus, light being the indicating force, we get chemical action on the plate, 
* Morean’s Electro-Physiology, p. 458. 
t+ Grove. The Correlation of the Physical Forces, 5th edition, p. 153. 
