ies of
jerks, an exposure being made automatically by a revolving shutter
during each rest. The positive film is placed in a lantern, and the
intermittent movement is repeated; but now the source of illumination is
behind the film, and light passes outwards through the shutter to the
screen. In the Urban bioscope the film travels at the rate of fifteen
miles an hour, upwards of one hundred exposures being made every second.

The impression of continuous movement arises from the fact that the eye
cannot get rid of a visual impression in less than one-tenth of a
second. So that if a series of impressions follow one another more
rapidly than the eye can rid itself of them the impressions will
overlap, and give one of _motion_, if the position of some of the
objects, or parts of the objects, varies slightly in each succeeding
picture.[25]

THE PLANE MIRROR.

[Illustration: FIG. 131.]

This chapter may conclude with a glance at the common looking-glass. Why
do we see a reflection in it? The answer is given graphically by Fig.
131. Two rays, A _b_, A _c_, from a point A strike the mirror M at the
points _b_ and _c_. Lines _b_ N, _c_ O, drawn from these points
perpendicular to the mirror are called their _normals_. The angles A
_b_ N, A _c_ O are the _angles of incidence_ of rays A _b_, A _c_. The
paths which the rays take after reflection must make angles with _b_ N
and _c_ O respectively equal to A _b_ N, A _c_ O. These are the _angles
of reflection_. If the eye is so situated that the rays enter it as in
our illustration, an image of the point A is seen at the point A^1, in
which the lines D _b_, E _c_ meet when produced backwards.

[Illustration: FIG. 132.]

When the vertical mirror is replaced by a horizontal reflecting surface,
such as a pond (Fig. 132), the same thing happens. The point at which
the ray from the reflection of the spire's tip to the eye appears to
pass through the surface of the water must be so situated that if a line
were drawn perpendicular to it from the surface the angles made by lines
drawn from the real spire tip and from the observer's eye to the base of
the perpendicular would be equal.

[22] Glazebrook, "Light," p. 157.

[23] Glazebrook, "Light," p. 157.

[24] Galileo was severely censured and imprisoned for daring to maintain
that the earth moved round the sun, and revolved on its axis.

[25] For a full account of Animated Pictures the reader might
advantageously consult "The