These mirrors furnish science with many curious and pleasing facts.

Concave mirrors are the reverse of convex; the latter being rounded outwards,
the former hollowed inwards--they render rays of light more converging--
collect rays instead of dispersing them, and magnify objects while the
convex diminishes them.

Rays of light may be collected in the focus of a mirror to such intensity as
to melt metals. The ordinary burning glass is an illustration of this fact;
although the rays of light are refracted, or passed through the glass
and concentrated into a focus beneath.

When incident rays are parallel, the reflected rays converge to a focus,
but when the incident rays proceed from a focus, or are divergent,
they are reflected parallel. It is only when an object is nearer to a
concave mirror than its centre of concavity, that its image is magnified;
for when the object is farther from the mirror, this centre will appear
less than the object, and in an inverted position.

The centre of concavity in a concave mirror, is an imaginary
point placed in the centre of a circle formed by continuing
the boundary of the concavity of the mirror from any one point
of the edge to another parallel to and beneath it.

REFRACTION OF LIGHT:--I now pass to the consideration of the passage
of light through bodies.

A ray of light failing perpendicularly through the air upon a surface
of glass or water passes on in a straight line through the body;
but if it, in passing from one medium to another of different density,