THE THEORY OF OPTICS.
Although the nature of light is absolutely unknown, and probably will long remain among the arcana of sci ence, yet philosophers have been led by the phenomena which it 'exhibits to suggest two hypotheses respecting its origin and propagation. lly one party, including Newton, and a great number of English philosophers, light is supposed to consist of material particles emitted by the luminous body with prodigous velocity, moving uniformly in straight lines, and capable of having its qualities and its direction altered by rcflexion from the surfaces of bodies, by refraction through their substance, and by inflexion in passing by their edges. By another party, including Descartes, Huygens, and Euler, light is supposed to resemble sound, and to consist in the undulations of a highly elastic fluid, diffused through all nature, and excited by the action of the luminous source.
During the last century, the Newtonian theory of lu minous particles was almost universally entertained both in England and on the Continent ; but the discoveries of the present day have revived the theory of Huygens, and it seems highly probable that this great question will speedily be set to rcst.
But whatever be the origin of light, and the mode of its propagation through space, it has certain general pro perties, which have been discovered by experiment, and are entirly independent of its nature.
1, Light is emitted in all directions, and from every .noint in the surface of 11(771i110118 and visible bodies.—This property of light is established by the fact, that wherever we are placed in relation to a luminous or visible body, every point of the part of the body turned towards us is visible. When the body is not self-luminous, the light by NS iliC it is visible must he that which, coming origi nally from a self-luminous source, is reflected from the surface and interior parts of the visible body.
2. Light consists of separate and independent parts.— A beam or mass of light flowing from a luminous body may be divided laterally into any number of smaller portions, which are called rays of light. These rays
may be supposed to be divided into smaller pnrtions in the direction of their length ; for we can stop light at any point or its path, and suffer the rest of it to proceed on its course. Hence We may with perfect propriety call one ol these portions of a ray of light a particle, or small portion of light, without considering whether it consists of actual matter, or is merely the undulation of an elastic medium.
5. Light, whether emitted directly fronz the sun, or re flected fronz the planets, nzoves at the rate of 195,000 miles per aecond.—This important fact was determined by Roemer, as we have already seen in our history of the science. See page 475.
4. Light, when nzoving in the same uniform medium, proceeds in a straight line.—The form of the shadows of bodies, ancl the impossibility of seeing objects through bent tubes, however small be their flexure, afford the most :Ample and palpable proof of this proper*, of light.
5. When light issues from a lunzinous point,and passes through a non-resisting and non-absorbing medium, its density at different distances front that point is inversely as the square of the distance.—If we suppose that the light from the luminous point passes through a rectan gular aperture at a given distance from thc point, and is received at a greater distance upon a white ground, the luminous image of the rectangular aperture will he a similar rectangle, and the triangles fortned by the rays passing by the ends or the homologous sides will be similar. Hence it follows, that the homologous sides will be to one another as their distances from the lu minous point ; and since the arcas are as the squares of the homologous sides, the areas will bc as the squares of their distances ; but as the densities are inversely as the areas, they will also be inversely as the squares of the distances.