Very good microscopes may be made with the crystalline lenses of
fish, birds, and quadrupeds. As the lens of fishes is spherical or
spheroidal, it is absolutely necessary, previous to its use, to
determine its optical axis and the axis of vision of the eye from which
it is taken, and place the lens in such a manner that its axis is a
continuation of the axis of our own eye. In no other direction but this
is the albumen of which the lens consists symmetrically disposed in
laminæ of equal density round a given line, which is the axis of the
lens; and in no other direction does the gradation of density, by which
the spherical aberration is corrected, preserve a proper relation to
the axis of vision.
When the lens of any small fish, such as a minnow, a par, or trout,
has been taken out, along with the adhering vitreous humour, from
the eye-ball by cutting the sclerotic coat with a pair of scissors,
it should be placed upon a piece of fine silver-paper previously
freed from its minute adhering fibres. The absorbent nature of
the paper will assist in removing all the vitreous humour from
the lens; and when this is carefully done, by rolling it about
with another piece of silver-paper, there will still remain,
round or near the equator of the lens, a black ridge, consisting
of the processes by which it was suspended in the eye-ball. The
black circle points out to us the true axis of the lens, which
is perpendicular to a plane passing through it. When the small
crystalline has been freed from all the adhering vitreous humour,
the capsule which contains it will have a surface as fine as a
pellicle of fluid. It is then to be dropped from the paper into a
cavity formed by a brass rim, and its position changed till the
black circle is parallel to the circular rim, in which case only
the axis of the lens will be a continuation of the axis of the
observer’s eye.--_Edin. Jour. Science_, vol. ii.