5 
on the Structure of Nerves . 
« tissus cellulaires ? ou de ce que la pulpe de la retine est trop 
“ amoncel^e, et empeche les rayons de lumiere d’arriver jusqu’a 
“ ces memes nerfs ?”* 
In considering the use of the lucid tapetum, it was an idea 
of the late Mr. Hunter’s, that the retina received a double 
stroke from the rays of light which entered the eye ; one in 
passing to the tapetum, the other in returning from it. 
This very ingenious opinion had some difficulties opposed to 
it, while the retina was supposed capable of obstructing the 
rays of light even in the smallest degree, as they could not be 
equably transmitted, so as to affect every part of the membrane 
alike. But the retina being ascertained to be absolutely trans- 
parent, these objections are entirely removed, and there can 
be no doubt that the rays of light, in those eyes which have a 
lucid tapetum, must remain upon the retina as long again as in 
the eyes of other animals; since the time required to strike 
upon the tapetum, and return, must be twice as much as is ne- 
cessary for passing through the retina, to reach the nigrum 
pigmentum, where they are lost. 
This may appear to be a consideration of little consequence, 
as the velocity of light is so great, and the continuance of im- 
pression necessary for distinct vision is that produced by a suc- 
cessive flow of similar rays of light from the object ; it may, 
however, be all that is necessary for the purpose. 
The retina being found perfectly transparent, when the eye 
is examined in a recent state, led to the idea that the internal 
structure of the optic nerve, when examined in the same state, 
might also be transparent. To ascertain this point, the fol- 
lowing experiment was made : 
* Sur le Venin de la Vipere, 1781. Vol. II. p. 219. 
