49 
on the Mechanism of the Eye. 
distance of this intersection from the retina was 637 thousandths. 
This nearly corresponds with the former calculation ; nor can 
the distance of the centre of the optic nerve from the point of 
most perfect vision be, on any supposition, much less than that 
which is here assigned. And, in the eyes of quadrupeds, the 
most strongly painted part of the choroid is further from the 
nerve than the real axis of the eye. 
I have endeavoured to express in four figures, the form of 
every part of my eye, as nearly as I have been able to ascertain 
it; the first (PI. V. Fig. 17.) is a vertical section; the second 
(Fig. 18.) a horizontal section ; the third and fourth are front 
views, in different states of the pupil. (Fig. 19 and 20.) 
Considering how little inconvenience is experienced from so 
material an inequality in the refraction of the lens as I have 
described, we have no reason to expect a very accurate provision 
for correcting the aberration of the lateral rays. But, as far as 
can be ascertained by the optometer, the aberration arising from 
figure is completely corrected; since four or more images of the 
same line appear to meet exactly in the same point, which they 
would not do if the lateral rays were materially more refracted 
than the rays near the axis. The figure of the surfaces is some- 
times, and perhaps always, more or less hyperbolical* or ellip- 
tical: in the interior laminae indeed, the solid angle of the 
margin is somewhat rounded off; but the weaker refractive 
power of the external parts, must greatly tend to correct the 
aberration arising from the too great curvature towards the 
margin of the disc. Had the refractive power been uniform, it 
might have collected the lateral rays of a direct pencil nearly as 
well; but it would have been less adapted to oblique pencils of 
* Petit Mem. del* Acad. 1725, p. 20. 
H 
MDCCCI. 
