3 
on the Structure of Nerves . 
in which the experiments were made was so much darkened 
that nothing but the illumination from the eye was visible, the 
animal, by change of posture, or some other means, almost 
immediately deprived the observers of all light from that source. 
This was found to be the case, whether the cat, the tiger, or 
the hyena, was the subject of the experiment. On the other 
hand, when the light in the room was sufficient for the animal 
itself to be seen, the illumination in the eye was more obscure, 
and appeared to arise from the external surface of the iris. 
As the difficulties which occurred in making observations 
on the illuminated state of the eye in the living animal were so 
great, an attempt was made to repeat, as nearly as possible, 
the experiments after death. 
In doing so it was found, that a strong light thrown upon 
the cornea illuminated the iris, as it had done in the living eye; 
but, when the cornea was removed, this illumination disap- 
peared. The iris was then dissected off, and the lucid tapetum 
completely exposed to view; the reflection from which was 
extremely bright ; the retina proving no obstruction to the rays 
of light, but appearing equally transparent with the vitreous 
humour and crystalline lens. 
From these experiments it appeared evident, that no light is 
generated in the eye ; the illumination being wholly produced 
by the concave bright-coloured surface of the tapetum, col- 
lecting the rays of the external light, concentrated by the 
cornea and crystalline lens, and reflecting them through the 
pupil. When the iris is completely open, the degree of bril- 
liancy is the greatest ; but, when the iris is partly contracted, 
which it always is when the external light is increased, then 
the illumination is more obscure, and appears to come from the 
B 2 
