TION and REFRACTION.
That great author put it past doubt, that all those rays which are reflected do not really touch the body, though they approach it infinitely near; and that those which strike on the parts of solid bodies adhere to them, and are, as it were, ex tinguished and lost. Since the reflection of the rays is ascribed to the action of the whole surface of the body without contact, if it be asked how it happens that all the rays are not reflected from every surface, but that, while some are reflected, others pass through and are refracted ? the answer given by Newton is as follows : Every ray of light, in its passage through any refracting surface, is put into a certain transient constitution or state, which in the progress of the ray returns at equal intervals, and disposes the ray at every return to be easily trans mitted through the next refracting sur face, and between the returns to be easily reflected by it : which alteration of reflec tion and transmission, it appears, is pro pagated from every surface, and to all distances. What kind of action or dis position this is, and whether it consist in a circulating or vibrating motion of the ray, or the medium, or something else, lie does not inquire ; but allows those who are fond of hypothesis to suppose that the rays of light, by impinging on any fleeting or refracting mike, excite vi brations in the reflecting or. refracting medium, and by that means agitate the solid parts of the body. These vibrations, thus produced in the medium, move fast er than the rays, so as to overtake them ; and when any ray is in that part of the vibration which conspires with its motion, its velocity is increased, and so it easily breaks through a refracting surface ; but when it is in a contrary part of the vibra tion, which impedes its motion, it is easily reflected ; and thus every ray is succes sively disposed to be easily reflected or transmitted by every vibration which meets it. These returns in the disposi tion of any ray to be reflected, he calls fits of easy reflection; and the returns in the disposition to be transmitted, he calls fits of easy transmission ; also the space between the returns, the interval of the fits. Hence then the reason why the sur
faces of all thick transparent bodies re flect part of the light incident upon them, and refract the rest, is, that some rays, at their incidence, are in fits of easy reflec tion, and others of easy transmission. For the properties of reflected light, see Mt Ft• ROR, OPTICS, Etc.
Again, a ray of light passing out of one medium into mother of different density, and in its passage making an oblique angle with the surface that separates the mediums, will be refracted, or turned out of its direction ; because the rays are more strongly attracted by a denser, than by a rarer medium. That these rays are not refracted by striking on the solid parts of bodies, but that this is effected without a real contact, and by the same force by which they are emitted and reflected, only exerting itself differently in different circumstances, is proved, in a great mea sure, by the same arguments by which it is demonstrated that reflection is per formed without contact.
When light is refracted by a prism, or other transparent body, it is divided into rays, exciting the sensation of different colours ; namely, red, orange, yellow, green, blue, indigo, and violet. This is the enumeration tollowed by Newton and others, which supposes seven rays re frangible in the above order, the red be ing least refrangible, and the violet most so, and that the other tints are produced by mixture. The image formed by the different rays, thus separated; forms the solar spectrum. Dr. Wollaston has shown, by looking through the prism at a narrow line of light, that the primitive colours are only red, green, blue, and violet.
Heat and light are not present in cor responding degrees, in different parts of the solar spectrum ; for, generally speak ing, those rays illuminate most that have the least heating power. The rays in the centre of the spectrum have the greatest illuminating power, as may be ascertain ed by viewing, successively in each, a small body, such as the head of a com mon nail. It will be seen most distinctly in the light green, or deep yellow rays, and less plainly towards either extremity of the spectrum.