40 
Dr . Young’s Lecture 
frequently taken notice of a similar circumstance ; that many 
persons were obliged to hold a concave glass obliquely, in order 
to see with distinctness, counterbalancing, by the inclination of 
the glass, the too great refractive power of the eye in the direc- 
tion of that inclination, (Cor. 10. Prop. IV.) and finding but 
little assistance from spectacles of the same focal length. The 
difference is not in the cornea, for it exists when the effect of 
the cornea is removed by a method to be described hereafter. 
The cause is, without doubt, the obliquity of the uvea, and of 
the crystalline lens, which is nearly parallel to it, with respect 
to the visual axis : this obliquity will appear, from the dimen- 
sions already given, to be about 10 degrees. Without entering 
into a very accurate calculation, the difference observed is found 
(by the same corollary) to require an inclination of about 13 
degrees ; and the remaining three degrees may easily be added, 
by the greater obliquity of the posterior surface of the crystalline 
opposite the pupil. There would be no difficulty in fixing the 
glasses of spectacles, or the concave eye-glass of a telescope, in 
such a position as to remedy the defect. 
In order to ascertain the focal distance of the lens, we must 
assign its probable distance from the cornea. Now the versed 
sine of the cornea being 1 1 hundredths, and the uvea being 
nearly flat, the anterior surface of the lens must probably be 
somewhat behind the chord of the cornea ; but by a very incon- 
siderable distance, for the uvea has the substance of a thin mem- 
brane, and the lens approaches very near to it : we will there- 
fore call this distance 12 hundredths. The axis and propor- 
tions of the lens must be estimated by comparison with anato- 
mical observations; since they affect, in a small degree, the de- 
termination of its focal distance. M. Petit found the axis 
