Illumination and Illuminating Apparatus 103 



of the eyepiece of the monocular tube. 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. 36) by 



E. and J. Beck. In this form 



each of the selenites is provided 



with a ring which rotates. The 



three being one over the other, 



either two or all three can be 



rotated together or in opposite 



directions to one another, and the effect is most striking. 



An inexpensive modification of this is made by Swift and 



Sons and others, called the mica-selenite stage, as shown 



in Fig. 37. This consists of a film of mica made to rotate 



FIG. 36. BARKER'S 

 SELENITES. 



FIG. 37. MICA-SELEXITE STAGE. 



in a brass plate, upon which the object is laid, and beneath 

 it is a carrier with three separate selenites. These can 

 each be pushed separately beneath the mica and the latter 

 rotated. By this means all the different tints obtainable 

 with any number of selenite films can be produced. It can 

 be employed on any microscope. To get greater brilliancy 

 the polarizer can be made to fit into the sub-stage con- 



