246 PRACTICAL MICROSCOPY. 



optical tube, immediately under which, in the same way, 

 is fitted a Klein's quartz plate. An extra analysing prism 

 with divided circle is placed over the eye-piece, and a con- 

 trivance for rotating crystals between it and the prism is 

 also added. The eye-piece has spider lines accurately 

 adjusted to the plane of the prism. 



When the Nicol prisms (polariser and analyser) are set 

 at right angles to each other they are said to be crossed, 

 and there is a total extinction of light passing to the eye of 

 the observer; a thin section of an amorphous substance, 

 such as glass, if placed upon the stage, does not produce 

 any change, neither does a similar slice of any mineral 

 which crystallises in the cubic system ; and if the field 

 remains perfectly dark when an object is observed between 

 crossed " Nicols," it may be safely inferred that it belongs 

 to one of these classes, or happens to be lying in a plane 

 yielding single refraction only. 



Some crystals appear not to polarise until the stage upon 

 which the slide is laid be rotated, and if it becomes visible 

 by this treatment it is certain the crystal does not belong 

 to the cubic system. 



Minerals of all the other systems of crystallography 

 behave in various manners under crossed Nicols, and this 

 branch of study may be easily carried out with the aid of 

 the information to be found in Chapter VIII. Sections 

 should be cut in various planes of the crystal so as to 

 represent the principal directions of vibration and also the 

 axes of elasticity; but as this properly belongs to the 

 science of petrology, the student should refer to those 

 treatises in which these questions are specially considered. 



In the petrological microscope shown by Fig. 214 there 

 is fitted an arrangement for showing the rings in biaxial 

 crystals : a crystal of augite spar (diopside) is brought into 

 the field of the microscope, and its entire system of rings 



