Chapter III.

The _eyes of insects_ are sometimes allowed to dry in their natural shape, and mounted as opaque objects; but generally they are used as transparencies in balsam or liquid, so the description of the treatment which they require will be deferred to Chapter III.

Hairs, when not too dark, are sometimes transparent enough when mounted dry, but are usually mounted in balsam. These also will be more fully noticed in another place.

These are a few of the objects which are often mounted dry, but some of them should be shown in balsam or liquid also, and there is much difference of opinion as to the best way of preserving others. This, however, is explained by the transparency which the balsam gives interfering with one property of the object and yet developing another which would have remained invisible if preserved dry. The only method of overcoming this difficulty is to keep the object mounted in both ways, which is comparatively little trouble.

I may here mention that many prefer the lieberkuhn for the illumination of opaque objects; and a good background is gained by putting upon the under side of the slide, immediately beneath the object, a spot of black varnish, which does not interfere materially with the light.