181 
of Edinburgh, Session 1872 - 73 . 
6. We are therefore now in a position to state that the law of 
the variation in the electro-motive force of the retina and optic 
nerve holds good in the following groups of the animal kingdom : 
mammalia, aves, reptilia, amphibia, pisces, and Crustacea. 
7. Many experiments have been made which prove that the 
psycho-physical law of Fechner, alluded to in previous communi- 
cations, is not dependent only on perception in the brain, but in 
part on the structure of the eye itself. The effects which occur on, 
during, and after the action of light on the retina, also take place 
after the eye has been removed from all connection with the brain. 
Thus the law of Fechner is not, as has been hitherto supposed, a 
function of the brain alone, but is really a function of the terminal 
organ, the retina. 
8. We have also employed a new method of registering galvano- 
metrical variations, which may be of service in many physical and 
physiological researches. This consists in placing at the proper 
distance from the galvanometer, instead of the ordinary graduated 
scale, the surface of a cylinder covered with paper, and moving on a 
horizontal axis by clock-work. The spot of light reflected from the 
galvanometer mirror is rendered more precise by having the shade 
of the galvanometer lamp blackened over the entire surface, with 
the exception of a spot about three millimeters in breadth, in the 
centre of which a line or. cross is made of soot. The image of this 
line or cross is, of course, reflected by the mirror upon the cylinder. 
When the cylinder is set in motion by the clockwork, the spot of 
light may be accurately followed by the hand of the observer, after 
a little practice, with a fine brush moistened with ink. The cylin- 
der we employed performed a complete revolution in 80 seconds. 
This time was divided into 4 equal parts, each representing 20 
seconds, by 4 lines drawn transversely at equal intervals across the 
paper on the cylinder. The first space, between lines 1 and 2, re- 
presented 20 seconds, in which the eye was in the dark, and in 
which the electro-motive force is represented by a straight line; 
the second space, between lines 2 and 3, represented 20 seconds, 
during which the effect of the impact of light took place, and in 
which the variation of the electro-motive force is indicated either by 
tal Palace Aquarium, who supplied us with three specimens of Eledone (a 
cuttle-fish, to represent mollusea), but none arrived alive. 
