Page:Optics.djvu/203

 molecules, and the sides of it are only the different sides of these molecules. We must therefore necessarily conclude that these have faces endowed with different physical properties, and that in the present case the first reflexion turns towards the same point of space, faces, if not similar, at least endowed with similar properties. This arrangement of the molecules Malus denominated the Polarization of light, assimilating the operation of the first glass to that of a magnet which turns the poles of a number of needles all in the same direction.

Hitherto we have supposed that the incident and reflected rays made angles of 35° 25′ with the glasses: it is indeed only under that angle that the phænomenon takes place completely. If while the first glass remains fixed, the inclination of the second to the ray be ever so little altered, it will be found that the second reflexion will not be entirely destroyed in any position, though it will still be at a minimum in the east and west plane. If again the inclination of the ray to the second glass, being preserved, that on the first be changed, it will be seen that the ray will never pass entirely through the second glass, but the partial reflexions which take place at its surfaces are at a minimum in the above-mentioned position.

Similar phænomena may be produced by means of most transparent substances besides glass. The two planes of reflexion must always be at right angles, but the angle of incidence varies with the substance. According as the refracting power of this is greater or less than that of the ambient medium, the angle of polarization, measured from the surface, is greater or less than half a right angle. We have seen that for glass this angle is 35° 25′: for sulphate of barytes it is only 32°, and for diamond only 23°. If glass plates be placed in essential oil of turpentine which has a refracting power almost exactly equal to that of glass, the angle of polarization will be found to differ very little indeed from 4535 [sic]°. The reflexion at the second surface is supposed to take place on the ambient medium which bounds the glass. In general, according to an ingenious remark of Dr. Brewster's, the angle of polarization is characterised by the reflected ray being perpendicular to the refracted. The angles calculated on this hypothesis agree singularly well with experiment, and also confirm the rule given above for the different magnitudes of them, as will easily appear from Figs. ,

Errata