Dr. Young's Lecture 
48 
distances from the cornea to the perforation made by the nerve, 
and their difference was exactly one-fifth of an inch. To this 
we must add a fiftieth, on account of the eccentricity of the 
pupil in the uvea, which in the eye that I measured was not 
great, and the distance of the centre of the nerve from the 
point opposite the pupil will be 1 1 hundredths. Hence it ap- 
pears, that the visual axis is five hundredths, or one-twentieth of 
an inch, further from the optic nerve than the point opposite the 
pupil. It is possible that this distance may be different in dif- 
ferent eyes : in mine, the obliquity of the lens, and the eccen- 
tricity of the pupil with respect to it, will tend to throw a direct 
ray upon it, without much inclination of the whole eye ; and it 
is not improbable, that the eye is also turned slightly outwards, 
if looking at any object before it, although the inclination is 
too small to be subjected to measurement. 
It must also be observed, that it is very difficult to ascertain 
the proportions of the eye so exactly as to determine, with cer- 
tainty, the size of an image on the retina ; the situation, curva- 
ture, and constitution of the lens, make so material a difference 
in the result, that there may possibly be an error of almost one- 
tenth of the whole. In order, therefore, to obtain some confir- 
mation from experiment, I placed two candles at a small dis- 
tance from each other, turned the eye inwards, and applied the 
ring of a key so as to produce a spectrum, of which the edge 
coincided with the inner candle ; then, fixing my eye on the out- 
ward one, I found that the spectrum advanced over two-sevenths 
of the distance between them. Hence, the same portion of the 
retina that subtended an angle of seven parts at the centre of 
motion of the eye, subtended an angle of five at the supposed 
intersection of the principal rays; (Plate III. Fig. 11.) and the 
