Page:A treatise on optics.djvu/14

(5.) Light moves in straight lines, and consists of separate and independent parts, called rays of light. If we admit the light of the sun into a dark room through a small hole, it will illuminate a spot on the wall exactly opposite to the sun, — the middle of the spot, the middle of the hole, and the middle of the sun, being all in the same straight line. If there is dust or smoke in the room, the progress of the light in straight lines will be distinctly seen. If we stop a very small portion of the admitted light, and allow the rest to pass, or if we stop nearly the whole light, and allow only the smallest portion to pass, the part which passes is not in the slightest degree af- fected by its separation from the rest. The smallest portion of light which we can either stop or allow to pass is called a ray of light.

(6). Light moves with a velocity of 192,500 miles in a second of time. It travels from the sun to the earth in seven minutes and a half. It moves through a space equal to the circumference of our globe in the 8th part of a second, a flight which the swiftest bird could not perform in less than three weeks.

(7). When light falls upon any body whatever, part of it is reflected or driven back, and part of it enters the body, and is either lost within it or transmitted through it. When the body is bright and well polished like silver, a great part of the light is reflected, and the remainder lost within the silver, which can transmit light only when hammered out into the thinnest film. When the body is transparent, like glass or water, almost all the light is transmitted, and only a small part of it reflected. The light which is driven back from bodies is reflected according to particular laws, the considera- tion of which forms that branch of optics called catoptrics ; and the light which is transmitted through transparent bodies is transmitted according to particular laws, the consideration of which constitutes the subject of dioptrics.