POLARIZATION OF LIGHT. A ray of light from the sun or a lamp, which has not been reflected or refracted in its course to the eye, possesses no properties by which one aide of it can be distinguished from another; if, for instance, it be divided into two by a colorless doubly-refracting crystal, such as Iceland spar, these two rays will be of appar ently equal intensity in whatever position the crystal be placed (liEnucTioN, DounLE). But if the ray has been reflected from a surface of glass or water, it is found that in general the intensities of the two rays into which it is divided by the doubly-refracting crystal are not only unequal, but dependent upon the position of the crystal with refer ence to the plane in which the light was previously refracted or reflected. This is a conclusive proof that the light has undergone some change by reflection or refraction, so that it is no longer the same all round, but possesses sides (iu the language of Newton), or (in modern phraseology) is polarized. Perhaps the most complete illustration of this very important fact is to be found by using two doubly-refracting bodies—two small crystals of Iceland spar, for instance—and pasting on a side of one of them a slip of paper with a pin-hole in it. On looking crystal, the covered side being turned toward a bright body, we see two images of the pin-hole, equally bright. Look at these through the second crystal, each is in general doubled; we see four images of the pin-hole, but these are generally unequal in brightness;* and by turning either of the Crystals round the line of sight as an axis, we find that there are positions, at right angles to each other, in which only two images are visible. If we turn further, the lost images appear faint at first, and gradually becoming brighter, while the others become fainter in proportion; till, when we have completed a quarter of a revolution, the acv images alone remain, the others having disappeared. From this it follows 'that each of the rays into which a single beam of light is decomposed by double refraction possesses aides, or is polarized; and to such an extent as to be incapable of being again doubly refracted in certain positions of the second crystal. By taking advantage of the differ ence of the refractive indices (REFRACTION) of the two rays produced by Iceland spar, and the close agreement of one of them with that of Canada balsam, Nicol constructed his "prism," which is one of the most useful pieces of polarizing- apparatus. It consists of two pieces of Iceland spar.cemented with Canada balsam, and allows only one of the two rays produced by double refraction to pass through. 'When we look at a flame through
two Nicol's prisms in succession, we find that the amount of light transmitted depends on their relative position. If they are similarly placed, we have the maximum amount —viz., half the incident light; if they are crossed, that is, if one be made to rotate through a right angle front the position last mentioned, no light, not even the most pow erful sunlight, can pass through the transparent combination. There are certain doubly refracting bodies, such as tourmaline, iodosulphate of quinine, etc., which by absorption stifle one of the two rays into which they divide a beam of light, and which act there fore precisely as Nicol's prism does. But they have the great disadvantage of coloring the transmitted light very strongly; and this renders them unfit for the study of the gorgeous phenomena of color (perhaps the grandest displays in optics) which are pro duced by polarized light. But for the verification of the facts to which we now proceed, a tourmaline or a Nicol's prism will do equally well, and will be called the analyzer. And first as to the reflection of light, a cause of polarization first detected by Malus. If we examine by the analyzer light reflected from water, unsilvered glass, polished or var nished wood, jet, etc., we find that it is more or less completely polarized; but that there is a particular angle for each substance, at which if light be reflected (see REFLEC TION) from its surface it is completely polarized; that is, can be completely stopped by the analyzer in certain positions, just as a ray which has passed through a Nicol's prism. It was discovered by Brewster that this angle, called the polarizing angle, has its tan gent equal to the index of refraction of the reflecting body: or, in another form, the reflected light from a surface of glass, water, etc., is completely polarized when its direc tion is perpendicular to tlmt of the corresponding refracted ray. The light reflected from the second surface of a glass plate is also completely polarized at the same angle; and one of the most useful polarizers which can be made is a pile of thin glass plates, from the surfaces of which light is reflected at the proper angle, which is for ordinary window-glass about 54°. The light which passes through the glass plates is partially polarized, and its polarization is more nearly complete the greater the number of plates employed. And it appears that these rays are polarized in planes perpendicular to each other—i.e., that the analyzer which extinguishes the reflected ray has to be turned through 90° to extinguish the refracted ray.