is called the _emergent_ or refracted ray. If
the eye is placed at T, and a bright object at P, the object is seen not at
P, but at the point H, since the eye cannot follow the course taken by the
refracted rays. In other words, objects viewed through a prism always
appear deflected towards its summit.

[Illustration: FIG. 66.]

In considering the action of a lens we can regard any lens as being built
up of a number of prisms with curved faces in contact. Such a lens is shown
in Fig. 67, the light rays being refracted towards the base of the prisms
or towards the normal, as already explained; while the top half of the lens
will refract all the light downwards, the bottom half will act as a series
of inverted prisms and refract all the light upwards.

[Illustration: FIG. 67.]

[Illustration: FIG. 68.]

If a beam of parallel light--such as light from the sun--be passed through
a double convex lens L, Fig. 68, we shall find that the rays have been
refracted from their parallel course and brought together at a point F.
This point F is {130} termed the principal focus of the lens, and its
distance from the lens is known as the focal length of that lens. In a
double and equally convex lens of glass the focal length is equal to the
radius of the spherical surfaces of the lens. If the lens is a plano-convex
the focal length is twice the radius of its spherical surfaces. If the lens
is unequally convex the focal length is found by the following rule:
multiply the two radii of its surfaces and divide twice that product by the
sum of the two radii, and the quotient will {131} be the focal length
required. Conversely, by placing a source of light at the point F the rays
will be projected in a parallel beam the same diameter as the lens. If,
however, instead of being parallel, the rays proceed from a point farther
from the lens than the principal focus, as at A, Fig. 69, they are termed
divergent rays, but they also will be brought to a focus at the other side
of the lens at the point a. If the source of light A is moved nearer to the
principal focus of the lens to a point A^1 the rays will come to a focus at
the point _a_^1, and similarly when the light is at A^2 the rays will come
to a focus at the point _a_^2. It can be found by direct experiment that
the distance _fa_ increases in the same proportion as AF diminishes, and
diminishes in the same proportion as AF increases. The relationship which
exists between pairs of points