I ICROSC ' ! WFSTIC ATION 



very feebly so, while liornblende is strongly 



Poland /tfA/.'By means of polarised light an 

 exceeding e method of investigation 



available. Hut for it* adequate use tome 

 with optics is necessary. For the methods of research 

 reference should be made to the treatises quoted on 

 p. 150, 



To examine minerals and rocks in polarised light both 

 the Nicol prisms are employed in their respective places. 

 If the object examined is a piece of glass, or an amor- 

 phous body, or a crystal belonging to some substance 

 .stallises in the regular or cubic system, the 

 light will reach our eye unaffected by the on of 



the object, when the axes 01 prisms are at right 



angles. The field will remain dark, in the same way as if 

 no intervening object were there. If, however, the sub- 

 stance under examination is a mineral belonging to one 

 of the other crystallographic systems, it will modify the 

 polarised beam of light On rotating one of the prisms 

 we perceive bands or flashes of colour, and numerous 

 lines appear which before were invisible. The field 

 no longer remains dark when the two Nicol prisms are 

 crossed 



It is evident, therefore, that we may readily tell by 

 this means whether or not a rock contains any glassy 

 constituent If it does, then that portion of its mass 

 will become dark when the prisms are crossed, while the 

 crystalline parts (except those belonging to the isometric 



1 On thi subject the Undent iboakl 



