47 
' on the Mechanism of the Eye. 
vision. The next three curves show the images formed after 
the refraction at the anterior surface of the lens, distinguished in 
the same manner ; and the three following, the result of all the 
successive refractions. The tenth curve is a repetition of the 
ninth, with a slight correction near the axis; at F, where, from 
the breadth of the pupil, some perpendicular rays must fall. By 
comparing this with the eleventh, which is the form of the re- 
tina, it will appear that nothing more is wanting for their perfect 
coincidence, than a moderate diminution of density in the lateral 
parts of the lens. If the law, by which this density varies, were 
more accurately ascertained, its effect on the image might be 
calculated from the eighth proposition ; but the operations would 
be somewhat laborious : probably the image, thus corrected, 
would approach very nearly to the form of the twelfth curve. 
To find the place of the entrance of the optic nerve, I fix 
two candles at ten inches distance, retire sixteen feet, and direct 
my eye to a point four feet to the right or left of the middle of 
the space between them : they are then lost in a confused spot 
of light ; but any inclination of the eye brings one or the other 
of them into the field of view. In Bernoulli’s eye, a greater 
deviation was required for the direction of the axis ; * and the 
obscured part appeared to be of greater extent. From the 
experiment here related, the distance of the centre of the optic 
nerve from the visual axis is found (by Prop. V.) to be 16 hun- 
dredths of an inch ; and the diameter of the most insensible part 
of the retina, one-thirtieth of an inch. In order to ascertain the 
distance of the optic nerve from the point opposite to the pupil, 
I took the sclerotica of the human eye, divided it into segments, 
from the centre of the cornea towards the optic nerve, and ex- 
tended it on a plane. I then measured the longest and shortest 
# Comm, Petrop. I. p, 314. 
