147 



VII.— POLARIZERS FOR THE MICROSCOPE. 



By B. K. Johnson, Department of Optical Engineering, 

 Imperial College of Science and Technology. 



{Bead March 16, 1921.) 



Iceland Spar is very scarce and is becoming more scarce. In the 

 highest class of petrological microscopes, designed for advanced 

 and research work, a form of Mcol polarizer made of Iceland Spar 

 is desirable, but it is not necessary for the simpler students' 

 microscopes. A very efficient polarizer can be made by taking an 

 ordinary 3x1 slip and flowing a little black varnish over one 

 surface and then allowing it to dry. Such a slip can then be 

 stuck to the plane reflector with soft wax and tilted by trial to the 

 polarizing angle. In the case of a tilting microscope either sky- 



s,<.y 



.v*^ 



rt"f. 



Light reflected from a bundle of plates varying in number from 1 to 32, the 

 light being incident at the polarizing angle (56|^). 



o 



Number op "PuATes, 



