LIGHT 611 In the last-mentioned year Malus, while engaged on the theory of double refraction, casually examined through a doubly refracting prism of quartz the sunlight reflected from the windows of the Luxembourg palace. He was surprised to find that the two rays alternately disappeared as the prism was rotated through successive right angles, in other words, that the reflected light had acquired properties exactly corresponding to those of the rays transmitted through Iceland spar. Even Malus was so imbued with the corpuscular theory of light that he named this phenomenon polarization, holding it as inexplicable on the wave theory, and as requiring a species of polarity (akin to the magnetic) in the light-corpuscles a close reproduction of one of Newton s guesses. ans- But after a short time Hooke s old guess was independ- rse ently reproduced, and in the hands of Young and others, ^ ra " but most especially of Fresnel, the consequences of the assumption, that the vibrations of the luminiferous medium take place perpendicularly to the direction of the ray, were the almost complete explanation of the cause of double refraction, and the discovery (often the prediction) of a long series of the most gorgeous phenomena known to science. The real difficulty in the way of this conception probably lay in the fact that most of the familiar forms of wave- motion such as sound-waves in air or in water, and ordinary water waves are not of this character. In sound-waves the vibrations are wholly in the direction of the ray, while in surface-waves in water they are partly parallel to and partly perpendicular to the direction in which the wave is travelling. That a body may transmit waves in which the vibration is perpendicular to the direction of a ray, it must have the properties of an elastic solid rather than of a fluid of any kind. And our experience of the almost entire absence of resistance to the planetary motion seems, at first sight at least, altogether incompatible with the idea that the planets move in a jelly-like solid, filling all space through which light can be propagated. alogies. Without going into difficult dynamical details, we may obtain a notion of the nature of the motion now to be considered, by observing the propagation of a wave when a long stretched wire or string is struck or plucked near one end. Here the line of motion of each part of the wire is almost exactly perpendicular to the direction of the wire, i.e. to the line along which the wave travels. (When the string is extensible there may be another wave, due to extension ; but this, which is analogous to sound, has its vibrations along the string, and it usually travels at a very different rate from the other, so that the two are not in any way associated). Now it is clear that waves of this wholly transverse character can have, in Newton s language, sides. And it is also clear that they cannot interfere so as mutually to destroy one another unless their corresponding sides are parallel to one another ; nor can they interfere at all if their sides are perpendicular to one another. Hence a very severe test of the theory will be furnished by examining various cases of interference of polarized light, which ought to present in general marked differences from those of ordinary light. It was by experiments of this kind that Fresnel and Arago first firmly established the bases of the theory of polarization. The important fact discovered by Malus was soon generalized into the following state ment : Light reflected from the surface of substances so different as water, glass, polished wood, &c., at a certain definite angle, which depends on the nature of the substance, is found to possess all the properties of one of the rays transmitted through Iceland spar. If the plane of re flexion is parallel to the axis of the spar, the properties of the reflected light are those of the ordinary ray ; if perpendicular to it, those of the extraordinary ray. It was reserved for Brewster to discover, as the resul of an extraordinary series of experimental measurements, the very simple law which follows : The tangent of the polarizing angle is equal to the Brewster s refractive index of the reflecting substance. ^ aw - This may be put in another form, in which its connexion with theory is a little more evident : When the reflected ray is completely polarized, it is perpendicular to the refracted ray. Bearing in mind Huygens s observations on light which has passed through two crystals of Iceland spar, we can now see that a ray of light polarized by reflexion is in general divided into two by a crystal of Iceland spar. But there is only one ray when the principal plane of the crystal is parallel to the plane of reflexion, and none when these planes are perpendicular to one another. We may now much simplify matters by suppressing the Polarizer Iceland spar, and using two reflecting plates of glass, so aiul placed that a ray meets each of them in succession at the anal y sei polarizing angle. It is then found that when the planes of reflexion are parallel the ray is reflected (almost without loss) from the second plate, but when they are perpendicular to one another there is complete extinction. In intermediate positions the intensity was found by Arago to be as the square of the cosine of the inclination of these planes. This very simple experiment, which any one may easily make for himself, by putting two pieces of glass at the proper angle in the ends of two wooden tubes which fit into one another, enables us to form a general notion of the modification which is called polarization. The " sides " of the reflected ray are obviously in, and perpendicular to, the plane of incidence ; for a ray can be reflected over and over again if the successive planes of incidence are parallel, but is stopped at once if one of them be perpendicular to the others. Here, however, two new difficulties come in at once : Diffi- (1) Are the vibrations of the reflected ray in, or perpen- culties dicular to, the plane of reflexion ] (2) As ordinary sun or m t] lamp light, reflected at the proper angle from a polarizing surface, shows no variation of intensity when the azimuth of the plane of reflexion is changed, what can be then the direction of its vibrations 1 These questions have not yet been answered in a thoroughly satisfactory manner Many important phenomena are explained in terms quite Plane of independent of the proper answer to (1); and, in others vibra- which do depend on the answer, the theoretical differences tion between the results of the two hypotheses are so small polariza- as to have hitherto remained undetected. In an important tion. test, suggested by Stokes, the experimental results have been at variance in a way not yet explained. It is quite possible that, as is required by Clerk Maxwell s electro magnetic theory of light (see ETHER), there may be simul taneous displacements, but of different characters, in each of these planes, and then the question would be reduced to Which of these displacements is the luminous one? But on this theory, loth are probably essential to vision. As to the second question, it may be said flrst, that, Nature so far as the test of double refraction can inform us, a of com polarized ray whose plane of polarization is made to rotate I? , 11 , rapidly produces precisely the same effects as a ray of ordinary light ; and, secondly, that, so great is the number of vibrations even of red light in one second, it would be impossible to make the plane of polarization rotate fast enough to affect the circumstances of any of the phenomena of interference, even when they take place between two portions of the same ray, one of which is retarded
