104 



THE MICROSCOPE. 



doms which defy all such modes of examination, and which will yield 

 only to the magical analysis of polarised light. A body which is 

 quite transparent 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 mine- 

 ral, vegetable, and animal structures, the extraordinary organisation 

 of Apophyllite and Analcime may be referred to ; also the symmetrical 

 and figurate depositions of siliceous crystals in the epidermis of equi- 

 setaceous plants, and the wonderful variations of density in the crystal- 

 line lenses of the eyes of animals. 



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

 ture 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 reflected 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, provided 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 pro- 

 perties 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 abed, fig. 69 ; and we shall 



Otf 



fig. 69. 



distinguish the section of a beam of common light by a circle with two 

 diameters a b, c d, at right angles to each other. 



If we now allow the same beam of light to fall upon a rhomb of 

 Iceland spar, and examine the two circular beams, Oo Ee, formed by 

 double refraction, we shall find, 1st, that the beams o E e have dif- 

 ferent properties on different sides, so that each of them differs in this 

 respect from the beam of common light. 



