102 



Modern Microscopy 



through the trough and its contents. A spot lens has a 

 longer focus, and gives under these circumstances the best 

 results. 



The Polariscope. 



This consists of two parts, each composed of a Nicol 

 prism of Iceland spar in a suitable mounting — one called 

 the polarizer, which fits into the sub- stage, and the other , 

 the analyzer, which is usually inserted between the nose- 

 piece of the microscope and the objective. By its means 

 light is split up into its component parts, and most beauti- 

 ful colour effects are obtained. The polarizer has a flange 

 beneath, by which it can be rotated, and in this way the 



Fig. 34. — Polakizer. 



Fio. 35. — Analyzee. 



colours are varied. In examining certain chemical crystals, 

 geological slides, etc., it brings into view structure which 

 without it would hardly be detected, and for this it is 

 largely used in analytical work. In some instruments the 

 analyzer prism is fitted in the body. This is rather an in- 

 convenience unless the instrument be designed especially 

 for petrology. For a binocular microscope, however, if it is 

 placed between the nosepiece and the objective, it causes a 

 separation between these two, which interferes with the 

 performance of the binocular prism ; because the closer the 

 back lens of the objective can be brought to the binocular 

 prism, the more perfect will, the vision be. Under these 

 circumstances the monocular tube only is generally used ; 

 or the analyzer prism can be mounted over the top 



