Page:A treatise on optics.djvu/13

A TREATISE ON OPTICS.

(1). Optics, from a Greek word which signifies to see, is that branch of knowledge which treats of the properties of light and of vision, as performed by the human eye.

(2). Light is an emanation, or something which proceeds from bodies, and by means of which we are enabled to see them by the eye. All visible bodies may be divided into two classes — self-luminous and non-luminous.

Self-luminous bodies, such as the stars, flames of all kinds, and bodies which shine by being heated or rubbed, are those which possess in themselves the property of discharging light. Non-luminous bodies are those which have not the power of discharging light of themselves, but which throw back the light which falls upon them from self-luminous bodies. One non-luminous body may receive light from another non-lumi- nous body, and discharge it upon a third ; but m every case the light must originally come from a self-luminous body. When a lighted candle is brought into a dark room, the form of the flame is seen by the light which proceeds from the flame itself; but the objects in the room are seen by the light which they receive from the candle, and again throw back ; while other objects, on which the light of the candle does not fall, receive light from the white ceiling and walls, and thus become visible to the eye.

(3). All bodies, whether self-luminous or non-luminous, dis- charge light of the same color with themselves. A red flame or a red-hot body discharges red light; and a piece of red cloth discharges red light, though it is illuminated by the white light of the sun. .

(4). Light is emitted from every visible point of a luminous or of an illuminated body, and in every direction in which the point is visible. If we look at the flame of a candle, or at a sheet of white paper, and magnify them ever so much, we shall not observe any points destitute of light.