The leaf at the side of the above picture is intended to give an idea of the mode of cutting out the designs, and in this case the leaf would be cut and bent back, and a small attachment slip of cardboard left to prevent it falling out.
The cardboard design is always bent toward the light, which is placed behind it. As a good illustration of the importance of reflected light and its connexion with luminous bodies, a beam of light from the oxy-hydrogen lantern may be allowed to pass above the surface of a table, when it will be noticed that the latter is lighted up only when the beam is reflected downward by a sheet of white paper.
By reference to the two laws of reflection already explained, it is easy to trace out on paper, with the help of compasses and rule, the effect of plane, concave, and convex surfaces on parallel, diverging, or converging rays of light, and it may perhaps assist the memory if it is remembered that a plane surface means one that is flat on both sides, such as a looking-glass: a convex surface is represented by the outside of a watch-glass; a concave surface, by the inside of a watch-glass; parallel rays are like the straight lines in a copy-book; diverging and converging rays, are like the sticks of a fan spread out as the sticks separate or diverge; the sticks of the fan come together, or converge at the handle.
The reflection of rays from a plane surface may be better understood by reference to the annexed diagram. (Fig. 262.)
Fig. 262.
a i, a k. Two diverging rays incident on the plane surface, d. a d is perpendicular, and is reflected back in the same direction. a i is divergent, and is thrown off at i l. The incident and reflected rays forming equal angles, as proved by the perpendicular, h. Any image reflected in a plane mirror appears as far behind it as the object is before it, and the dotted lines meeting at g show the apparent position of the reflected image behind the glass, as seen at g. The same fact is also shown in the second diagram, where the reflected picture, i m, appears at the same distance behind the surface of the mirror as the object, a b, is before it.
By the proper arrangement of plane mirrors, a number of amusing delusions may be produced, one of which is sometimes to be met with in the streets, and is called "the art of looking through a four-inch deal board." The spectator is first requested to look into a tube, through which he sees whatever may be passing the instrument at the time; the operator then places a deal board across the middle of the tube, which is cut away for that purpose, and to the astonishment of the juveniles the view is not impaired, and the spectator still fancies he is looking through a straight tube; this however is not the case, as the deception is entirely carried out by reflection, and is explained in the next cut. (Fig. 263.)
Fig. 263.