120 



THE MICROSCOPE 



In the tubular portion connecting the lamp to the illuminator there 

 are two slots into which colour screens, ground glass, or a focussing 

 lens can be dropped, and a ground glass, a green glass, and a 

 lens are supplied with the microscope for the purpose. 



Fig. 111. — No. 3226, Metallurgical Bench Microscope with prism 

 illuminator and electric lamp and fittings. 



Pefcroiogicai A pctrological microscopc is essentially an ordinary microscope 

 microscopes. pjQyj(jg(j ^j^j^^ ^ number of special adjustments and appliances 

 for the study of rocks and crystals. The most important of these 

 additions are a polarising apparatus and a rotating stage. A 

 polarising apparatus consists of a Nicol prism made of Iceland 

 spar which must be placed below the object to be examined and 

 a similar but somewhat smaller prism which must be placed 

 above the object. At least one of the prisms and the object 

 must be capable of rotation, and the amount of the rotation 

 determined on a scale. There must be a means of rapidly throwing 



