if by any process or under any circumstances this compound ray gets split up into its component parts, then we can imagine the two rays proceeding on their journey independent of each other. In this state no difference would be observed without special means, to be mentioned hereafter, of observing them. Both the rays are said to be polarized. Polarization is thus defined by Professor Whewell :— "Opposite properties in opposite directions so exactly equal as to be capable of accurately neutralizing one another." Light may be polarized by reflection, and by single and double refraction. FIRST, BY REFLECTION. A B Fig. 64. Let CA (Fig. 64) represent a ray of light falling upon the mirror A, it is reflected to mirror B; and if this were in a perpendicular position, the ray would be reflected from the second mirror. It is, however, at the polarizing angle of glass, viz., 56°, and it will give no reflection. From the first mirror A there is a reflected ray, but it is polarized, that is, it consists of light in one plane only, and when this strikes the second mirror at the polarizing angle 50° 45', it is not reflected at all. Light is much more commonly polarized than is generally supposed. The surface of smooth water, slates on the roofs of houses, especially when wet, polarize light by reflection. To ascertain if it be so, it is only necessary to examine it by an analyzer, as it is called. In two positions the analyzer will reflect no light if the observer is experimenting on light already polarized. SECOND, BY SINGLE REFRACTION. If a ray of light passes through a series of glass plates, it is polarized by refraction. The incident angle at which the ray must fall to produce polarization varies according to the number of plates. For 20 plates this angle is about 65°. A smaller number of plates will partially polarize the light, but the effect of these plates is never very satisfactory. THIRD, BY DOUBLE REFRACTION. Double Refraction is a property which some bodies possess of causing a ray of light, in passing through them, to undergo two refractions; that is, a single ray is divided into two rays. Many crystals have this power, but Iceland spar possesses it in a remarkable degree. That the phenomenon of double refraction is due entirely to the molecular structure of the medium through which the light passes, is proved by taking a cube of regularly annealed glass, which produces but one refracted ray; but on heating the glass and subjecting it to pressure, or to rapid cooling, a change is effected in the molecular arrangement of the parts, and double refraction takes place. B Fig. 65. In Fig. 65 let A DE C represent the section of a crystal of Iceland spar, and let a ray of light proceed from letter B; it will be doubly refracted; the ordinary ray will pass to G, and an extraordinary ray will pass to H. The eye will see the letter B in positions and d, the ordinary refraction producing b, the extraordinary d. The ordinary laws of refrac tion do not apply to the latter ray. If we look at a small object, such as a letter, through a plate of glass, it appears single, but if a plate of Iceland spar be substituted a double letter will be seen. Turn the crystal round, and the two images will be seen apart, the extraordinary image revolving round the ordinary. Both these rays are polarized in planes at right angles to each other. For the compound ray in its course meets with varying resistance in its two planes from the structure of the crystal, and as this is the same as elasticity, and velocity depends upon elasticity, therefore the rays cannot emerge together. A crystal of tourmaline cut in two, and the two parts placed at right angles with each other, will allow no light to pass through them, although separately they are transparent. One plate will act as polarizer, and the other as analyzer. To understand this on the theory already advanced, a b Fig. 66. с let a, Fig. 66, represent the tourmaline polarizer, along which in the direction of the axis we will suppose lines to be drawn ; these will represent the obstruction which prevents the passage of light polarized in one plane; b is the same placed crosswise to represent the hindrance to the passage of the light polarized in a plane at right angles to the first; and shows clearly that all light will be prevented by placing the crystals with their axes across each other. A small instrument called the Tourmaline Pincette may be used for demonstrating the above. It consists of a bent wire in the form of pincers, in the two ends of which are placed small discs of wood, moveable; and in the centres of the discs are placed the tourmaline crystals. Suppose the eye to be placed so as to observe light passing through, then allow either of the two to remain at rest as a polarizer, and turn the other-the analyzer-round slowly. It will be found that in two positions light is transmitted, and in other two they are opaque. Let us begin with a clear transmission of light at oo, then this will lessen, and it will gradually be reduced up to 90°, when no light will pass. Ágain it will become gradually lighter up to 180°, then dark again at 270°, and, of course, again clear at 360°. From the preceding explanation this will be understood by the careful reader. Noremberg's Polariscope is the most simple and complete apparatus for experiments with polarized light. It consists of a disc of wood with a circular mirror fitted in it; from the disc rise two supports, in the lower part of which is a reflecting glass plate, hung as a common swing looking-glass. There is at the side a graduated circle, by which to set the glass at any desired angle. At the top of the supports a wooden ring is placed in which may be put a Nicol's prism, or a darkened reflector may be supported at an angle, and made to revolve. It is used in the following manner :-A ray of light strikes the reflecting glass at the polarizing angle. One part of the ray is thrown off polarized, while the other polarized ray passes through the glass to the mirror. From this it is reflected vertically to the top of the framework, where the analyzer may be brought to bear upon it. This apparatus will readily show the alternate light and dark positions, as mentioned under the tourmaline pincette. Amongst the many most interesting and instructive experiments on light and colours which may be performed by means of the polariscope, the following is mentioned as an illustration : Place a very thin plate of selenite or mica on a support between the unsilvered mirror and the analyzer, so that the polarized ray, in rising to the top, must pass through the crystalline film. We will suppose this to be of a monochromatic colour, say red; then, as the analyzer is turned round, the colour will disappear, and at 45° there will be no light transmitted; then, on turning again, the complementary colour green will show up to 90°, and again at 135 all will be dark; turn further, and red will again appear at 180°; and so on round the whole circle. Light is also polarized by reflection from many substances, such as glass, water, ebony, air, motherof-pearl, and the surfaces of crystals, provided that the light fall at a certain angle peculiar to each substance; and this angle is called the polarizing angle. The best polarizer is a Nicol's prism. Fig. 67. 1654 extraordinary ray is 1.483 Canada balsam is 1.549 A crystal of Iceland spar is cut across from obtuse angle A to obtuse angle B, the two half-crystals are then cemented together with Canada balsam. The ordinary ray is so refracted by structure of the crystal |