Dr. Young’s Lecture 
7 ° 
they appear to me, although he had received no hint of what 1 
had observed. The lateral refraction is the most easily ascer- 
tained, by substituting for the slits a tapering piece of card, so 
as to cover all the central parts of the pupil, and thus deter- 
mining the nearest crossing of the shadows transmitted through 
the marginal parts only. When the furthest intersection was at 
38, I could bring it to 22 parts with two narrow slits ; but with 
the tapered card only to 2 9. From these data we may deter- 
mine pretty nearly, into what form the lens must be changed, 
supposing both the surfaces to undergo proportional alterations 
of curvature, and taking for granted the dimensions already 
laid down : for, from the lateral aberration thus given, we may 
find (by Prop. III.) the subtangents at about one- tenth of an 
inch from the axis ; and the radius of curvature at each vertex, 
is already determined to be about 21 and 15 hundredths of an 
inch. Hence the anterior surface must be a portion of a hyper- 
boloid, of which the greater axis is about 50 ; and the posterior 
surface will be nearly parabolical. In this manner the change 
will be effected, without any diminution of the transverse dia- 
meter of the lens. The elongation of its axis will not exceed 
the fiftieth of an inch ; and, on the supposition with which we 
set out, the protrusion will be chiefly at the posterior vertex. 
The form of the lens thus changed will be nearly that of Plate V. 
Fig. 26 ; the relaxed state being nearly as represented in Fig. 25. 
Should, however, the rigidity of the internal parts, or any other 
considerations, render it convenient to suppose the anterior surface 
more changed, it would still have room, without interfering 
with the uvea ; or it might even force the uvea a little forwards, 
without any visible alteration of the external appearance of 
the eye. 
