LIGHT 445 creasing or destroying each other according to whether they conspire together or not. At a certain distance from 0, as at F, the difference of the paths of the two beams is equal to a half undulation, and the waves interfere with and destroy one another, producing a dark ringA At a greater distance, as at G, the dif- ference of the paths is equal to one whole undulation; therefore the waves conspire to- gether and increase the amount of light, which also is decomposed, producing rainbow colors. With homogeneous light there is simply alter- nate increase of light and darkness. Now as the thickness of the plate of air at the distance from the centre where any ring is formed is in proportion to the square of this distance, or to the square of the diameter of the ring, it is only necessary to measure these diameters in order to determine the law of thickness. This was done by Newton with great accuracy, who found that the squares of the diameters were in an arithmetical progression; consequently the thicknesses of the plates corresponding to the successive rings form a similar progression. He moreover measured the absolute thickness of the plate of air at which each ring was formed, and found that at the dark ring of the fifth order it was TY/SU-O f an inch, this being ten times the thickness at the first bright ring. The successive orders of bright rings are there- fore formed at the thicknesses TT~SYOO~O> TTsfoTru"? TrrlWr* & c -> an( l the intermediate dark rings at the thicknesses T^?^, iT?o<nn TY?<nn>, &c. POLARIZATION OF LIGHT. While making his researches on the law of double refrac- tion, Huygens found that the rays divided by passing through a rhomb of Iceland spar pos- sessed remarkable properties; that on passing them through a second rhomb the two portions did not remain equally intense; that their rela- tive brightness depended on the position of the second rhomb, and that there were two posi- tions in which one of the rays completely dis- appeared. Each of the rays has therefore ac- quired characteristic properties, or, it may be said, has lost properties. It is said to have ac- quired sides, but in fact it is reduced to vibra- tions in one plane. At Newton's suggestion it was said to be polarized. The phenomenon discovered by Huygens remained for more than a century an isolated fact in science, and other phenomena with which it is associated also re- mained unnoticed till the beginning of the present century. In 1808, while Malus was en- gaged in researches upon the subject of double refraction, he happened to turn a double-re- fracting prism toward the windows of the Lux- embourg palace, which then reflected the rays of the setting sun. On turning round the prism the ordinary image of the window nearly dis- appeared in two opposite positions, while in two others at right angles to the former the extraordinary image nearly vanished. He at first ascribed this phenomenon to some influ- ence of the atmosphere upon the ray similar to that produced by a second rhomb of Ice- land spar, but he soon found that it was caused by reflection at a particular angle. This there- fore was the second important discovery in re- gard to polarization : that it was not only pro- duced by transmission, but by reflection; a discovery of great value in investigating the FIG. 18. properties of light. A*beam of light from a self-luminous source when passing through a homogeneous medium exhibits the same prop- erties on all sides so long as it does not meet with an obstacle; such a beam is composed of ordinary or natural light. But after it has been reflected or refracted, it has lost some of its properties; some of its rays have been quenched. When the reflection takes place at a certain angle, nearly all the rays except those lying in a certain plane will have been obliter- ated. If the ray, having been thus reflected from a glass mirror, be received obliquely upon another glass mirror, and the latter turned around the ray, care being taken not to change the angle of incidence, the intensity of the twice-reflected beam will vary as the position of the mirror is changed. Let a ray of light A 0, fig. 13, fall upon a plate of glass at 0, making an angle of 54 35' with the perpen- dicular; it will be reflected in the direction D. Let the ray be received upon a second plate of glass, at the same angle with the per- pendicular as before. If the second mirror is so placed that its plane of reflection is parallel to the plane of reflection from the first surface, it will be reflected in the direction D E with- out being diminished. But if the second mir- FIG. 14. ror has its plane of reflection perpendicular to that of the first, as in fig. 14, then the ray will not be reflected, or its intensity will be greatly reduced. In intermediate positions at the same angle of incidence it will be partly reflected, the quantity of light being greater in propor-
