242 PRACTICAL MICROSCOPY. 



His improved apparatus may be seen in Fig. 212, having 

 been reduced from the drawing which appeared in the 

 ' Monthly Microscopical Journal/ 



The real advantage of the apparatus (if any) seems to 

 be in the application of gearing to produce revolution of 

 the films, an operation easily performed by the fingers 

 alone in the Ross instrument, as well as those of other 

 makers. 



The colours produced by the selenite films are brought 

 about by variations in their thickness, the red being 

 yielded by the thickest, violet by the thinnest ; these films 

 vary between "00124 and *oi8i8 of an inch. The arrow- 

 head and letters PA indicate the positive axis of the 

 selenite, while the figures , f , and f denote the parts of a 

 vibration retarded by each disc. 



But the student need not go to much expense over these 

 elaborate stages. They are intended to save time and not 

 money, and the ordinary plain mounted selenite films on a 

 3x1 inch glass slide will answer many purposes, though 

 incapable of rotation. If the red and green, blue and 

 yellow slides be provided, with a film of mica, this is about 

 all the student need purchase, at all events for some time 

 to come. 



Rock crystal or quartz possesses the extraordinary 

 power called rotatory polarisation. If a ray of homo- 

 geneous light be polarised, and the analyser turned so that 

 it appears totally extinguished, the ray can be made to 

 pass again through the analyser by placing a thin section 

 of quartz, with its plane at right angles to the rays, 

 between the polariser and analyser. In order to effect 

 this the quartz crystal must be cut at right angles to its 

 axis. 



In most petrological microscopes of the present day 

 Klein's plate of quartz is inserted in a movable box placed 



