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.
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.
Chapter XIV.
SOUND AND MUSICAL INSTRUMENTS.
Nature of sound—The ear—Musical instruments—The vibration of strings—The sounding-board and the frame of a piano—The strings—The striking mechanism—The quality of a note.
Sound differs from light, heat, and electricity in that it can be propagated through matter only. Sound-waves are matter-waves, not ether-waves. This can be proved by placing an electric bell under the bell-glass of an air-pump and exhausting all the air. Ether still remains inside the glass, but if the bell be set in motion no sound is audible. Admit air, and the clang of the gong is heard quite plainly.
Sound resembles light and heat, however, thus far, that it can be concentrated by means of suitable lenses and curved surfaces. An echo is a proof of its reflection from a surface.
Before dealing with the various appliances used for producing sound-waves of a definite character,