416 PROCEEDINGS OF SECTION D. 
lens would be most marked, and the mechanical arrangement of 
the ocular muscles, have received exhaustive study, with the most 
satisfactory results. 
The essential act of vision is, however, the stimulation of the 
rods and cones by light. Of this part of the process, however, 
little knowledge has been gained. Here, again, we are stopped 
from want of mechanical analogy as a guide to investigation. The 
process is now intracellular, and the methods employed up to this 
point seem useless wherewith to press further. 
The rods and cones are sensitive to light; so are the pigment 
cells in the skin of a frog. In both cases we judge this by result; 
in the frog’s pigment corpuscles the light falling upon them is 
followed by a dissipation of energy in the movement of these 
processes ; in the case of the rods and cones, the result is the 
passage of a nerve impulse—a process of which we know little 
beyond the rate with which it travels along a nerve, and that it 
is associated with the dissipation of a small amount of electrical 
energy. ‘The relationship of the one to the other is a question to 
be attempted in the future. 
As a third example, I will briefty mention the nature of the 
evidence to prove that the law of conservation of energy holds for 
the living animal machine just as it does for any piece of ordinary 
mechanism. This I think one of the triumphs of the physiology 
of the last fifty years. Ever since the principle of the conservation 
of energy was established, physiologists have been endeavouring to 
ascertain whether this held good for animals, and whether an energy 
balance-sheet could be constructed for animals just as has been 
done by physicists for heat-engines. 
The income of energy is in the form of the potential energy of 
the food stuffs taken ; the output mostly heat and mechanical 
work, with a certain very small amount as sound and electrical 
energy. The potential energy set free by the oxidisation of the 
food taken can be determined so that the total incoming energy 
over any period can be ascertained. Again, by placing an animal 
in a calorimeter, the whole of the output of energy can be 
measured as heat. 
In some such way the applicability of the general law of the 
conservation of energy has been frequently tested, but the results 
have until recently been insufficiently accurate to balance. A 
physiological experiment of this kind is extremely complicated, 
and the possible sources of fallacy are very considerable. Many 
additional data have to be ascertained for the following reasons :— 
(1.) The whole of the food eaten does not enter the body at all. 
(2.) The whole energy of the food absorbed is not utilised, but 
much of it is excreted in a form still possessed of considerable 
potential energy. Consequently, the potential energy of the forces 
representing the unutilised portion of the food, and the energy 
