474 CHAPTER XXIV 



Those bodies which rotate the plane of polarization are said to be optically 

 active. 



Means of obtaining Plane Polarized Light. — All light reflected from a 

 plane surface is partially polarized, and for a certain angle a it is wholly 

 polarized. This occurs when tan a — index of refraction of the reflecting 

 substance (Brewster's law).^ This method of obtaining polarized light was 

 used in the first polarimeter of record by Biot,^ to whom the science of polari- 

 metr}^ in general, and its application to sugar analysis in particular, is due. 



The means now almost always adopted to obtain a ray of plane polarized 

 light is the prism of Nicol or some modification thereof. The construction 

 of this is explained below. 



If ordinary light be allowed to pass through a rhombic crystal of Iceland 

 spar, A BCD, Fig. 285, it suffers double refraction and two rays of light 

 emerge where only one entered. This phenomenon, illustrated in Fig. 285, 

 always obtains unless the entrant ra}/ of light pass in a direction parallel to 

 the line joining two opposite obtuse angles, this direction being known as 

 the optic axis, and any plane containing the optic axis and perpendicular 

 to the face of the crystal is known as a principal plane. 



Fig. 286 



Of the two rays into which the entrant ray is divided, that ray KL more 

 refracted from the original direction is known as the ordinary ray, the ray 

 less refracted, KM, being the extraordinary ray. On emergence both rays 

 are found to be plane polarized and in directions at right angles to each 

 other. Thus as illustrated the extraordinary ray vibrates in the plane of 

 the paper, the ordinary ray vibrating at right angles thereto. In 1829, 

 NicoP published a paper, "On a Method of so far increasing the Divergence 

 of the two Rays in Calcareous Spar that only one Image is seen at a Time." 

 The means he adopted was the total reflection of the ordinary ray within 

 the crj'Stal, obtained as follows : — 



Let A BCD, Fig. 286, represent a section through a crystal of Iceland 

 spar divided* along B D, and let the two parts into which the crystal is 

 divided be united by a transparent cement, such as Canada balsam. Now 

 the index of refraction of the ordinary ray is 1-658, and that of the extra- 

 ordinary ray is 1-486. That of Canada balsam is 1-55. Hence when the 

 ordinary ray K L meets the film of balsam it is reflected in the direction L N, 

 and if the dimensions of the crystal, and the angle of incidence of the beam 

 of light be properly selected, it will pass out through the upper face of the 

 crystal and be lost when the exterior surface is blackened. The extra- 

 ordinary ray KM passes through the balsam with small change of direction 



* The means adopted by opticians are either sawing through the prism with a copper wire and emery, or grind- 

 ing away one half of the prism. Splitting is a process not employed. 



