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MOTION PICTURE PHOTOGRAPHY
water, which move forward without moving the water which composes them forward. This we know, because a boat floating upon water agitated by waves, does not move forward with the waves, but simply bobs up and down in the same spot. In the same manner, light waves pass through the ether without the ether moving forward in the direction of the waves. There is a difference in the light waves and the water waves, however; for while the waves in water move up and down only, the vibrations, or waves, which occur in the ether, take place in every conceivable direction — sideways as well as up and down. Figure 2 represents a cross-section of a ray of light in which may conceive that the wave or ray is vibrating back and forth in every direction within the limits of a circle.
Waves of light pass through any transparent medium, which may be air, glass, water, celluloid, amber, or any other substance through which we can see. As long as light travels in the same substance or medium, it goes forward in a straight line, but as soon as it strikes the surface of a different medium, it is deflected or bent at a slight angle, depending upon the nature of the substance, and does not bend again until it encounters another medium. This is called the rectilinear propagation of light, which simply means, as before stated, that in any particular medium — whether air, water or glass, light always travels in straight lines.
The principal sources of light are from objects heated to a high temperature. The most common source of light is, of course, the sun, which is a heavenly body incandescently hot. In the arc light, the light is emitted by the carbon tips heated to incandescency by the passing of the electric current. Incandescent lights give forth light because their filaments are heated by the passing of the electric current. Ordinary kerosene lamp flames are luminous, because of the hot particles of carbon in the flame. Bunsen burners and alcohol lamps give forth very little light, because there are no solid particles in their flames to be heated to incandescency. There are exceptions to this rule of light being accompanied by heat, such as the glow of the glow-worm, phosphorescence of phosphorus, and light from some kinds of electric discharges. These exceptions are not very well understood and are seldom of any use in connection with photography.
In the Cooper-Hewitt lamp, vapor of mercury is rendered incandescent by the passing of the electric current. A luminous
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