PHYSIOLOGICAL ACTION OF LIGHT. 165 
the two methods described above will be found in Plate XI. appended to this 
paper. 
As this paper embodies only the experimental results obtained from a general 
survey of the subject, leaving many minute and intricate points of difference 
between the result of the natural stimulus applied to the nervous structure of 
the eye and the effect of stimulating a motor nerve (that is, what is called a 
negative variation of the normal nerve current), we do not enter into any 
theoretical explanations regarding the cause of these peculiarities, but leave 
all such matters for discussion and further elaboration in the second portion of 
this paper, on which we are at present engaged. In the meantime we may 
succinctly summarize the results of this part. 
XVII.—Summary or Part I. oN THE PHYsioLoGicaL ACTION oF LIGHT. 
We have experimentally proved—(1), that the impact of light on the eyes of 
members of the following groups of animals, viz., Mammalia, Aves, Reptilia, 
Amphibia, Pisces, and Crustacea, produces a variation amounting to from three 
to ten per cent of the normal electro-motive force existing between the corneal 
surface and the transverse section of the nerve ; (2), that this electrical altera- 
tion may be traced into the brain ; (3), that those rays that we regard as most 
luminous produce the largest variation ; (4), that the alteration of the electrical 
effect with varying luminous intensity seems to follow very closely ratios given 
by the psycho-physical law of FECHNER ; (5), that the electrical alteration is due 
to the action of light on the retinal structure itself, as it is independent of the 
anterior portion of the eye, eliminating, therefore, the natural supposition that 
the contraction of the iris might produce a similar result ; (6), that it is possible, 
by experiment, to discover the physical expression of what is usually called in 
physiological language, fatigue ; and (7), that the method employed in this 
research may be applied to the investigation of the special organs of the other 
senses. 
[ APPENDIX. 
VOL. XXVII. PART I. va 
