Accessories. 57 



and the objective. By its means light is split up into its 

 component parts, and most beautiful colour effects are 

 obtained. The polarizer has a flange beneath, by means 

 of which it can be rotated, and in this way the colours are 

 varied. For certain chemical crystals, geological slides, 

 etc., it brings into view structure which without it would 

 hardly be detected, and it is largely used in analytical 

 work. In some instruments the analyzer prism is fitted in 

 the body. This is rather an inconvenience unless the 

 instrument be designed especially for petrology. For a 

 binocular microscope, however, it is better not to have it 

 between the nosepiece and the objective, as it creates a 

 distance between these two, which interferes with the per- 



FIG. 16. Polarizer. FIG. 17. Analyzer. 



formance of the binocular prism ; in fact, 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 analyzer prism is best mounted over the 

 top of the eyepiece and the monocular tube alone used. 

 For use with the polariscope, varieties of tints and a back- 

 ground of colour can be obtained by the employment of 

 selenite films. These, in the cheapest form, are mounted 

 in the same way as ordinary microscopic objects ; but a still 

 greater variety of effect can be obtained by having selenites 

 fitting into a carrier to come between the polarizer and the 

 stage in a sub-stage microscope. We illustrate one (Fig. 18) 

 by E. and J. Beck. In this form each of the selenites is pro- 

 vided with a ring which rotates. The three being one over 



