410 Abstract of' Dr Hope's Address' on 



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

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

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

 slice be placed unconformahly 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 I 

 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 unco7iformably, 

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

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

 penetrate the two plates. This is polarization produced by 

 double refraction. 



Now, all these modes of polarization have been recognised by 

 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 the 

 case of heat polarized by reflection and transmission^ for which 

 purposes, instead of glass, (which permits scarcely any non4 

 luminous heat to penetrate it), Mr Forbes employs plates of 

 mica, divided by a peculiar process into extremely thin laminae. 



But the analogies which he has established between light and 

 heat do not stop here. It has been found in the case of light, 

 that, when the two reflecting plates before spoken of, or the 

 two crystals, are placed in unconformable positions, so that 

 little or no light reaches the eye, we may, by interposing be- 

 tween the plates or the crystals a thin lamina of a doubly re- 

 fracting substance (such as mica) in a certain position (relatively 

 to its internal structure), cause a portion of light, which before 

 was incapable of reaching the eye, to become capable of so doing. 

 In other words, the polarized light, which at first was incapable 

 of reflection or transmission at the second plate or crystal, now 

 becomes capable of it; it has lost, to a certain extent, its charac- 

 ter of polarization, or it is said to be depolarized. 



Dr Hope stated, that he had seen this to be most completely 



