LENSES.
21
enter that side or emerge from it, when the source
of light is at b. It must not be inferred that noth¬
ing can be done except with lenses of a particular
sort. On the contrary very much can be done with
such poor lenses as are used in dark lanterns, and
are full of striæ and air bubbles. The lenses that
come with ordinary magic lanterns will answer for many
purposes. Spectacle glasses, linen provers, botanical
glasses, are all very useful. A pow¬
erful lens can be made out of two
watch glasses, one (a) a little larger
than the other (