ater. From
this we say that _the sines of the angles of incidence and refraction have
a constant proportion or ratio to one another_.

The number 1.336 is termed the _refractive index_, or _coefficient_, or the
_refractive power_ of water. The refractive power varies, however, with
other fluids and solids, and a complete table will be found in any good
work on optics.

Glass is the substance most commonly used for refracting the rays of light
in optical work, the glass being worked up into different forms according
to the purpose for which it {128} is intended. Solids formed in this way
are termed _lenses_. A lens can be defined as a transparent medium which,
owing to the curvature of its surfaces, is capable of converging or
diverging the rays of light passed through it. According to its curvature
it is either spherical, cylindrical, elliptical, or parabolic. The lenses
used in optics are always exclusively spherical, the glass used in their
construction being either crown glass, which is free from lead, or flint
glass, which contains lead and is more refractive than crown glass. The
refractive power of crown glass is from 1.534 to 1.525, and of flint glass
from 1.625 to 1.590. Spherical surfaces in combination with each other or
with plane surfaces give rise to six different forms of lenses, sections of
which are given in Fig. 65.

[Illustration: FIG. 65.]

All lenses can be divided into two classes, convex or converging, or
concave or diverging. In the figure, _b_, _c_, _g_ are converging lenses,
being thicker at the middle than at the borders, and _d_, _e_, _f_, which
are thinner at the middle, being diverging lenses. The lenses _e_ and _g_
are also termed meniscus lenses, and _a_ represents a prism. The line XY is
the axis or _normal_ of these lenses to which their plane surfaces are
perpendicular.

Let us first of all notice the action of a ray of light when passed through
a prism. The prism, Fig. 66, is represented by the triangle BBB, and the
incident ray by the line TA. {129} Where it enters the prism at A its
direction is changed and it is bent or refracted towards the base of the
prism, or towards the normal, this being always the case when light passes
from a rare medium to a dense one, and where the light leaves the opposite
face of the prism at D it is again refracted, but away from the normal in
an opposite direction to the incident ray, since it is passing from a dense
to a rare medium. The line DP