97 
crystallized bodies on homogeneous light. 
M. Biot, in the fourth vol. of his Traite de Physique, and he 
has availed himself of it with his accustomed ingenuity, as 
affording an extremely ready and convenient mode of viewing 
the phenomena of polarisation, much more so than by the use 
of plates of agate, prisms of Iceland spar, or a second re- 
flection. 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 po- 
larised 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 other per- 
pendicular to the axis, the former is extinguished in its pas- 
sage, 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. 
There is a great difference, however, in the degree in which 
tourmalines of different colours possess this power. Those 
of a light green, pink, or bluish colour, are quite improper, 
allowing a considerable portion of light to pass when so 
crossed, while, on the other hand, those whose colour verges 
strongly to the honey yellow, or to the hair brown, or pur- 
plish brown, effect nearly a complete absorption, and afford, 
when crossed, a combination almost impervious to light. In 
ignorance of this distinction, I sacrificed several fine and valu- 
able specimens before I could obtain proper plates. 
When a crystallized lamina, cut in a proper direction, is 
interposed between such a combination of plates, it disturbs 
the polarisation which the light has received in traversing 
the first plate, and renders a certain portion of it capable of 
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