C. Determination of Milk-Sugar. D. Determination of Cinchona Alkaloids. I. INTRODUCTION. § 1. In an ordinary ray of light the vibrations of the particles of ether take place successively in all possible directions perpendicular to its axis. Fig. 1 shows a transverse section of a ray, projected on a vertical plane. Fig. 1. Fig. 2. By certain means it is possible to restrict the vibrations to some one particular direction (Fig. 2). The ray is then called a linear polarized ray. Its behaviour is no longer identically the same all around its axis, as in an ordinary ray; on the contrary, it displays two distinct sides, one in the plane of its vibrations, the other in a plane at right angles thereto. § 2. This conversion of common into polarized light may be effected, first, by reflection, for which purpose a glass mirror, inclined to the perpendicular at a certain angle (35° 25′), as L M (Figs. 3 and 4), will be found best. Rays falling in the direction a b, so as to make an angle of 55° with the normal x y, are reflected in the direction bc, and at the same time are polarized. This becomes manifest when the reflected rays meet the mirror PQ, which has a rotatory movement about be as an axis. When the second mirror is parallel to the first, as in Fig. 3, the ray bc is wholly reflected in the direction cd; but as the mirror turns on its axis, the intensity of the light reflected from it diminishes, until at 90° from the starting-point there is no longer any reflection, and the mirror appears dark. Continuing the rotation, we find that at 180°, B 1 i.e., when the mirrors are inclined, so that the planes a b c and b c d are again coincident, as in Fig. 4, there is another maximum of reflection, and, lastly, at 270° another minimum. Thus the ray behaves differently in two different directions at right angles to each other, one direction being in the plane of incidence or reflection, a b c, the other in a plane at right angles therewith. The ray polarized, and the former of the two planes is its plane of polarization. is We can therefore recognize a ray as polarized, and determine the position of its plane of polarization, by permitting it to fall upon a glass mirror at an angle of 55°. If the mirror be turned about the ray as an axis, light and darkness will alternate at intervals of 90°, and if the mirror be set so that the light emitted by it is at its brightest, the plane passing through it and the incident polarized ray is the plane of polarization of the latter. Again, if the mirror is at its darkest, the plane at right angles to the plane of incidence of the polarized ray is coincident with the plane of polarization. By the undulatory theory of light it can be proved that the plane of polarization is either the plane in which the vibrations of ether take place, or a plane at right angles thereto. Which of these is really the case is still an open question among physicists; but for simplicity's sake we shall adopt the former view, and in these pages assume that the planes of vibration and polarization are coincident. § 3. But a pencil of light can also be polarized by repeated |