119 



two plates, complete reflection takes place at the second plate. 

 This illustrates polarization by reflection. 



If a number of glass plates be used, and light transmitted 

 obliquely through such a bundle of plates, it is in like manner 

 found, that the emergent light is wholly transmitted by a second 

 similar bundle placed parallel to the first, but is almost wholly 

 reflected, and therefore 7zo< transmitted, when the second bundle 

 is placed so that whilst the ray falls upon it at the same angle 

 as upon the first, the plane of incidence on the second bundle 

 being perpendicular to the plane of incidence upon the first 

 bundle. This is polarization by transmission or refraction. 



Lastly, It was observed before the close of the 17th century 

 by Huyghcns, that certain bodies, as Iceland spar, endowed with 

 the property of double refraction, alter at the same time the 

 character of the light in the two refracted rays. So that, if two 

 sections similarly cut from a crystal of Iceland spar be placed 

 upon one another in conformable positions, or the respective po- 

 sitions which they occupied on the crystal, the two rays will pro- 

 ceed through the second slice as they did through the first, and 

 be refracted according to the same laws. But if the second 

 slice be placed unconformuhli/ upon the first, or turned round a 

 quarter of a circle, the ray, which at first was ordinarily refract- 

 ed, is now extraordinarily refracted ; and the ray, which at first 

 was extraordinarily, is now ordinarily refracted. Now, it has 

 been found that some crystals, such as tourmaline, possess the 

 property, first, of dividing these rays, and then of suppressing 

 or absorbing one of them ; the result of which is, that when two 

 tourmalines, cut as we have supposed, are placed conformably^ 

 the ray which was not suppressed by the first slice, still makes 

 its way through the second ; but, when placed tmcoiifonnably, 

 the ray transmitted by the first plate is wholly suppressed by the 

 second. In the latter case, therefore, not a ray of litiht can 

 penetrate the two plates. This is polarization produced by 

 double refraction. 



Now, all these modes of polarization have been reco"-niscd bv 

 Mr Forbes in the case of heat, and even in the case of heat 

 wholly unaccompanied by light. The Vice-President announced 

 that he had witnessed this in the most satisfactory manner in tlie 



