160 MR JAMES DEWAR AND DR M‘KENDRICK ON THE 
1. The Cat.—The animal was securely fixed in CzERMAk’s rabbit-holder. 
The skin around the orbit was reflected. The zygomatic arch was snipped 
through by bone forceps, so as to expose as much as possible of the side of the 
orbit. The cellular tissue of the orbit was then pushed aside along the superior 
and the lateral aspect of the eyeball, so as to reach the optic nerve with as little 
disturbance as possible to the vascular arrangements of the eyeball. On exposing 
clearly the optic nerve, and staunching hemorrhage, the nerve was cut through 
transversely with sharp scissors. When this was done, the globe could be pulled 
downwards, inwards, and forwards, so as to expose a clear transverse section of 
the nerve. With the head firmly fixed, one narrow clay point was now placed 
on the cornea, while the other was in contact with the transverse section of the 
nerve. On opening the key a strong deflection was obtained, in several 
instances so powerful as to drive the spot of light off the scale. In one experi- 
ment it was for a long time impossible to obtain sufficiently slight contact to 
procure a deflection small enough to lie on the galvanometer scale. In several 
experiments we succeeded in tracing the curve made by the movement of the 
spot of light on the surface of a horizontal cylinder, and it will be seen, on 
referring to this curve, as shown in the annexed plate (Plate XI.), that on 
the impact of light the electro-motive force diminished, during its continuance 
the electro-motive force gradually rose to a point where it became steady, and 
on the withdrawal of light the inductive effect, that is, a rise in the electro-motive 
force, was well marked. 
2. The Pigeon.—The eye and optic nerve of the pigeon were more accessible 
than those of the cat, and the head of the bird was more easily held in a stable 
condition. The eye was found sensitive to light, and followed the same law as 
that of the cat. 
3. The Owl.—As this bird is believed to be capable of seeing objects with a 
very feeble intensity of light, we considered it important to examine its eye in 
the same manner as we examined that of the pigeon. We found exactly the 
same results. We could not detect any quantitative differences between the 
effects of light on the eye of the pigeon and on the eye of the owl. 
XJ.—EXPERIMENTS ON THE EFFECT OF LIGHT, IN WHICH A PORTION OF THE BRAIN WAS 
INCLUDED IN THE CIRCUIT. 
A number of experiments were made with the view of determining how far 
into the brain substance it was possible to observe any electrical variation pro- 
duced by the action of light on the retina, the optic nerve and the eyebail 
remaining in their natural position. 
1. The Frog.—On bisecting the head of a newly killed frog with a sharp 
pair of scissors, it is possible to obtain a longitudinal section of the brain, with 

