on the Mechanism of the Eye. 39, 
be thought that I assign too great a convexity to the cornea ; 
but I have corrected it by a number of concurrent observations* 
which will be enumerated hereafter. 
The eye being directed towards its image, the projection of 
the margin of the sclerotica is 22 hundredths from the margin 
of the cornea, towards the external angle, and 27 towards the 
internal angle of the eye: so that the cornea has an eccen- 
tricity of one fortieth of an inch, with respect to the section of 
the eye perpendicular to the visual axis. 
The aperture of the pupil varies from 27 to 13 hundredths ; 
at least this is its apparent size, which must be somewhat dimi- 
nished, on account of the magnifying power of the cornea, 
perhaps to 25 and 12. When dilated, it is nearly as eccentric 
as the cornea ; but, when most contracted, its centre coincides 
with the reflection of an image from an object held immediately 
before the eye ; and this image very nearly with the centre of 
the whole apparent margin of the sclerotica : so that the cornea 
is perpendicularly intersected by the visual axis. 
My eye, in a state of relaxation, collects to a focus on the 
retina, those rays which diverge vertically from an object at the 
distance of ten inches from the cornea, and the rays which 
diverge horizontally from an object at seven inches distance. 
For, if I hold the plane of the optometer vertically, the images 
of the line appear to cross at ten inches ; if horizontally, at seven. 
The difference is expressed by a focal length of 23 inches. I 
have never experienced any inconvenience from this imperfec- 
tion, nor did* I ever discover it till I made these experiments ; 
and I believe I can examine minute objects with as much accu- 
racy as most of those whose eyes are differently formed. On 
mentioning it to Mr. Cary, he informed me, that he had 
