JEWEL LENSES. 
power, one of glass and the other of diamond, the latter will have 
less convexity than the former. 
From what has been explained on the subject of spherical 
aberration, in our Tract upon Optical Images, it will be under¬ 
stood, that the more convex a lens is, the less its diameter must 
be, for if its diameter exceeds a certain limit relatively to its con¬ 
vexity, the spherical aberration will become so great as to render 
all vision with it confused and indistinct. This is the reason 
why all lenses of high magnifying power and short focal length 
are necessarily small. 
16. But since the spherical aberration depends on, and increases 
with the convexity of the lens, other things being the same, it 
follows that if two lenses, composed of different materials, have 
equal focal lengths, that which has the less convexity will also 
have less spherical aberration. 
17. Now, as according to what has been explained above, a 
diamond lens has less convexity than a glass lens of the same focal 
length, it will, if it had the same diameter, have less spherical 
aberration, or, what is the same, it will admit of being formed 
with a greater diameter, subject to the same aberration. 
18. In lenses of high magnifying powers, and which are con¬ 
sequently of small dimensions, any increase of the diameter which 
can be made without being accompanied with an injurious 
increase of aberration, is attended with the advantage of trans¬ 
mitting more light from each point of the object to the eye, and 
therefore of rendering the object more distinctly visible. It was 
on this account that, when single lenses of high magnifying 
power were thought desirable, great efforts were made to form 
them of diamond, and other transparent gems- having a refracting 
power greater than that of glass. 
19. Sir David Brewster, who first suggested the advantage of 
this, succeeded in getting lenses of great magnifying power, made 
of ruby and garnet; he considered those made from the latter 
stone to surpass every other solid lens: the focal length of some of 
those made for him was less than the l-30th of an inch, the mag¬ 
nifying power being more than 300. 
20. All these and similar efforts made by Messrs. Pritchard and 
Yarley, aided by the genius and science of the late Dr. Goring, 
have, however, happily for the progress of science, been subse¬ 
quently rendered unnecessary by the invention of methods of pro¬ 
ducing good achromatic object-glasses of high power for compound 
microscopes, so that the range of usefulness of simple microscopes, 
or magnifying glasses, is now limited to uses and researches in 
which comparatively low magnifying powers are sufficient. 
21. The most feeble class of magnifying glasses are those occa- 
