,i382] .] Crystallized Bodies on Homogeneous Light, Iw 



^nearly the whole, of an incident pencil polarised in a plane 

 parallel to that axis,"^ was pointed out by M. Biot, in the fourth 

 ^volume of his Traite de Physique, and he has availed himself of 

 iit with his accustomed ingenuity, as affording an extremely 

 iready and convenient mode of viewing the phenomena of polari- 

 sation, much more so than by the use of plates of agate, prisms 

 of Iceland spar, or a second reflection. It follows, from the 

 above-mentioned property, that if a beam of ordinary light be 

 .made to traverse such a plate, the whole of the emergent pencil, 

 •or nearly so, will be polarised in a plane at right angles to the 

 axis ; for the incident ray being divided by the doubly refracting 

 (force into two pencils, polarised in planes, the one parallel, the 

 cother perpendicular to the axis, the former is extinguished in its 

 ;j)assage, while the latter emerges with nearly its full intensity. 



Hence, if two such plates are crossed at right angles, though. 

 'Separately very transparent, their combination will be opaque. 

 lEhere is a great difference, however, in the degree in which tour- 

 jnalines of different colours possess this power. Those of a hght 

 ^reen, pink, or bluish colour, are quite improper, allowing a con- 

 <siderable portion of light to pass when so crossed ; while, on the 

 ^ther hand, those whose colour verges strongly to the honey 

 yellow, or to the hair brown, or purplish brown, effect nearly a 

 complete absorption, and afford, when crossed, a combination, 

 jftlmost impervious to light. In ignorance of this distinction, I 

 ^sacrificed several fine and valuable specimens before I could 

 obtain proper plates. 



When acrystaUized lamina, cut in a proper direction, is inter- 

 posed between such a combination of plates, it disturbs the 

 ^polarisation which the light has received in traversing the first 

 |>late, and renders a certain portion of it capable of traversing 

 the second: the colour and intensity of this portion, varying with 

 -the direction of the ray, give rise to the phenomena of the pola- 

 rised rings, which may accordingly be seen by applying the eye, 

 jSkiid receiving on it the dispersed hght of the clouds, &c. 



In order, however, to equalize as well as disperse the light, 

 iwhich is of great importance to obtaining a perfect view of the 

 'phenomena, an emeried glass may be cemented on the anterior 

 -plate, or the first surface of the plate itself roughened ; but it 

 will be found more convenient in practice to employ a double 

 45onvex lens of short focus for this purpose, by which, if neces- 

 sary, a very strong illumination may be obtained, and au 

 extremely minute portion of a crystal subjected to examination. 



* The same property is observable in the epidote, the axinite, and all other natural 

 and artificial crystals which exhibit any degree of dichroism when examined by nnpola-^ 

 riscd light. Muriate of palladium and potash possesses it in the highest perfection. 

 This remarkable effect is easily explained by a reference to the general principles laid 

 -^wn by Dr. Brevsterin his paper on absorption, Phil. Trans. 1819, p. 11. The inci« 

 •dent pencil is separated by the doubly refractive force into two, oppositely polarised, one 

 -of which is partly absorbed, the other emerges (polarised in its proper plane) of nearly its 

 original intensity. 



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