6 o 
Dr. Young’s Lecture 
increase of the length of the eye’s axis, the oval spot caused 
by the pressure would have spread over a space at least ten 
times as large as the most sensible part of the retina. But no 
such circumstance took place: the power of accommodation 
was as extensive as ever ; and there was no perceptible change, 
either in the size or in the figure of the oval spot. 
Again, since the rays which pass through the centre of the 
pupil, or rather the anterior vertex of the lens, may, as already 
observed, be considered as delineating the image ; and, since 
the divergence of these rays with respect to each other, is but 
little affected by the refraction of the lens, they may still be 
said to diverge from the centre of the pupil ; and the image of 
a given object on the retina must be very considerably en- 
larged, by the removal of the retina to a greater distance from 
the pupil and lens. (Cor. Prop. V*.) To ascertain the real 
magnitude of the image with accuracy, is not so easy as it at 
first sight appears; but, besides the experiment last related, 
which might be employed as an argument to this purpose^ there 
are two other methods of estimating it. The first is too hazard- 
ous to be of much use ; but, with proper precautions, it may be 
attempted. I fix my eye on a brass circle placed in the rays of 
the sun, and, after some time, remove it to the cancellated mi- 
crometer ; then, changing the focus of my eye, while the micro- 
meter remains at a given distance, I endeavour to discover 
whether there is any difference in the apparent magnitude of the 
spectrum on the scale ; but I can discern none. I have not insisted 
on the attempt ; especially as I have not been able to make the 
* This Corollary should stand thus. “ If a confused image be received on any 
given plane, it will be necessary, in order to determine its magnitude, to advert to the 
aperture admitting the rays. If the aperture be supposed to be infinitely small, it may 
be considered as a radiant point, in order to find the direction oi the emergent rays.’* 
