THE BIOSCOPE.
"Living pictures" are the most recent improvement in magic-lantern entertainments. The negatives from which the lantern films are printed are made by passing a ribbon of sensitized celluloid through a special form of camera, which feeds the ribbon past the lens in a series 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.
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 A1, in which the lines D b, E c meet when produced backwards.
Fig. 132.