52 
Dr. Young’s Lecture 
distances, appears to exist in very different degrees in different 
individuals. The shortest distance of perfect vision in my eye, 
is 2 6 tenths of an inch for horizontal, and 29 for vertical rays. 
This power is equivalent to the addition of a lens of 4 inches 
focus. Dr. Wollaston can see at 7 inches, and with converg- 
ing rays ; the difference answering to 6 inches focal length. Mr. 
Abernethy has perfect vision from 3 inches to 30, or a power 
equal to that of a lens inches in focus. A young lady of my 
acquaintance can see at 2 inches and at 4; the difference being 
equivalent to 4 inches focus. A middle aged lady at 3 and at 4 ; 
the power of accommodation being only equal to the effect of a 
lens of 12 inches focus. In general, I have reason to think, that 
the faculty diminishes in some degree, as persons advance in life; 
but some also of a middle age appear to possess it in a very small 
degree. I shall take the range of my own eye, as being probably 
about the medium, and inquire what changes will be necessary 
in order to produce it ; whether we suppose the radius of the 
cornea to be diminished, or the distance of the lens from the 
retina to be increased, or these two causes to act conjointly, or 
the figure of the lens itself to undergo an alteration. 
1. We have calculated, that when the eye is in a state of 
relaxation, the refraction of the cornea is such as to collect 
rays diverging from a point ten inches distant, to a focus at 
the distance of 13-j tenths. In order that it may bring to the 
same focus, rays diverging from a point distant 29 tenths, we 
find (by Cor. 5, Prop. IV.) that its radius must be diminished 
from 31 to 25 hundredths, or very nearly in the ratio of five 
to four. 
2. Supposing the change from perfect vision at ten inches to 
29 tenths, to be effected by a removal of the retina to a greater 
