POLARISED LIGHT. 143 



vegetable, and animal kingdoms, which defy all such modes 

 of examination, and which will yield only to the magical 

 analysis of polarised light. A body which is quite trans- 

 parent to the eye, and which might be judged as mono- 

 tonous in structure as it is in aspect, will yet exhibit, 

 under polarised light, the most exquisite organisation, and 

 will display the result of new laws of combination which 

 the imagination even could scarcely have conceived. In 

 evidence of the utility of this agent in exploring mineral, 

 vegetable, and animal structures, the extraordinary organi- 

 sation of Apophyllite and Analcime may be referred to ; 

 also the symmetrical and figurate depositions of siliceous 

 crystals in the epidermis of equisetaceous plants, and the 

 wonderful variations of density in the crystalline lenses of 

 the eyes of animals. 



Oo 



If we transmit a beam of the sun's light through a cir- 

 cular aperture into a darkened room, and if we reflect it 

 from any crystallised or uncrystallised body, or transmit 

 it through a thin plate of either of them, it will be reflcted 

 and transmitted in the very same manner, and with the 

 same intensity, whether the surface of the body is held 

 above or below the beam, or on the right side or left, pro- 

 vided that in all cases it falls upon the surface in the same 

 manner; or, what amounts to the same thing, the beam of 

 solar light has the same properties on all its sides; and 

 this is true, whether it is white light as directly emitted 

 from the sun, or from a candle or any burning or self- 

 luminous body; and all such light is called common light. 

 A section of such a beam of light will be a circle, like a b 

 c d y fig. 80 ; and we shall distinguish the section of a beam 



