334* Mr. Home's Account of an Orifice 
« of augmenting or diminishing the pupil, than in men? If 
“ Messrs. Mariotte and Le Cat should come to life again, 
“ they would find, in that hole, the explanation of the phaeno- 
“ menon of the two cards, one disappearing at a certain dis- 
“ tance from one eye, &c. which may be explained by saying, 
“ that where the optic nerve enters the ball, there is no cho- 
« roid, and so no vision. 
“ I dissected some human eyes a short time after I had read 
(t the discovery, and found the spot, the ruga concealing it, and 
“ the yellow zone. The best way, I think, to see them, is to take 
« off the half posterior part of the sclerotica, then the corre- 
“ spondent part of the choroid ; both must be cut round the 
« insertion of the optic nerve. The retina is to remain bare and 
“ untouched, sustaining alone the vitreous humour ; then you 
“ may see the round spot, which reaches the optic nerve, and a 
“ fold of the retina, marking a diameter of the spot. Then, if 
“ you press the ball a little with your finger, so as to push the 
« vitreous humour rather near the bottom of the eye, the ruga 
“ is unfolded, and you will see the hole perfectly round, of ~ of 
« a line in diameter, and its edges very thin. 
« All this can be seen on the inside of the eye, but not so 
« perfectly ; and, in that case, you must make your observa- 
“■tions in water.” 
Many months elapsed, after the receipt of this letter, beiore 
I could procure an eye in a proper state for observing this aper- 
ture in the retina ; but, in the course of last month, several 
opportunities offered, and I saw the appearance described by 
Mr. Maunoir very distinctly. 
The mode I adopted for examining the retina, was that of 
removing the transparent cornea; then taking away the iris. 
