THE PHONOGRAPH.--This instrument is the outgrowth of the stylus forms of the apparatus described, but in this case the stylus, or needle, is fixed to a metallic diaphragm, and its point makes an impression on suitable material placed on the outside of a revolvable cylinder or disc.
Light.-Light is the agent which excites the sensation of vision in the eye. Various theories have been advanced by scientists to account for the phenomenon, and the two most noted views are the _corpuscular_, promulgated by Sir Isaac Newton, and the _undulatory_, enunciated by Huygens and Euler.
The _corpuscular_ theory conceives that light is a substance of exceedingly light particles which are shot forth with immense velocity. The _undulatory_ theory, now generally accepted, maintains that light is carried by vibrations in ether. Ether is a subtle elastic medium which fills all space.
_Luminous_ bodies are those like the sun, which emit light. Rays may _diverge_, that is, spread out; _converge_, or point toward each other; or they may be _parallel_ with each other.
VELOCITY OF LIGHT.--Light moves at the rate of about 186,000 miles a second. As the sun is about 94,000,000 miles from the earth, it takes 8-1/2 minutes for the light of the sun to reach us.
REFLECTION.--One of the most important things connected with light is that of reflection. It is that quality which is utilized in telescopes, microscopes, mirrors, heliograph signaling and other like apparatus and uses. The underlying principle is, that a ray is reflected, or thrown back from a mirror at the same angle as that which produces the light.
When the rays of the sun, which are, of course, parallel, strike a concave mirror, the reflecting rays are converged; and when the rays strike a convex mirror they diverge. In this way the principle is employed in reflecting telescopes.
REFRACTION.--This is the peculiar action of light in passing through substances. If a ray passes through water at an angle to the surface the ray will bend downwardly in passing through, and then again pass on in a straight line. This will be noticed if a pencil is stood in a glass of water at an angle, when it will appear bent.
Refraction is that which enables light to be divided up, or analyzed. In this way white light from the sun is shown to be composed of seven principal colors.