38 
Dr. Young’s Lecture 
VI. Being convinced of the advantage of making every 
observation with as little assistance as possible, I have endea- 
voured to confine most of my experiments to my own eyes ; 
and I shall, in general, ground my calculations on the suppo- 
sition of an eye nearly similar to my own. I shall therefore 
first endeavour to ascertain all its dimensions, and all its 
faculties. 
For measuring the diameters, I fix a small key on each point 
of a pair of compasses ; and I can venture to bring the rings 
into immediate contact with the sclerotica. The transverse 
diameter is externally 98 hundredths of an inch. 
To find the axis, I turn the eye as much inwards as possible, 
and press one of the keys close to the sclerotica, at the exter- 
nal angle, till it arrives at the spot where the spectrum formed 
by its pressure coincides with the direction of the visual axis, and, 
looking in a glass, I bring the other key to the cornea. The 
optical axis of the eye, making allowance of three hundredths 
for the coats, is thus found to be 91 hundredths of an inch, from 
the external surface of the cornea to the retina. With an eye 
less prominent, this method might not have succeeded. 
The vertical diameter, or rather chord, of the cornea, is 45 
hundredths : its versed sine 1 1 hundredths. To ascertain the 
versed sine, I looked with the right eye at the image of the 
left, in a small speculum held close to the nose, while the left 
eye was so averted that the margin of the cornea appeared as a 
straight line, and compared the projection of the cornea with 
the image of a cancellated scale held in a proper direction be- 
hind the left eye, and close to the left temple. The horizontal 
chord of the cornea is nearly 49 hundredths. 
Hence the radius of the cornea is 31 hundredths. It may 
