44 



THE MICROSCOPIST. 



dered dark, and doubly refracting bodies on the stage of 

 the microscope appear either illuminated or in colors. If 

 a polarized ray passes through a doubly refracting film, 

 as of selenite, it forms two distinct rays, the ordinary 

 and the extraordinary ray. Each of these will be of dif- 

 ferent colors, according to the thickness of the film. If 

 one be red, the other will be green, these colors being 

 complementary. By using the analyzer one of these rays 

 is alternately suppressed, so that on revolving the appa- 

 ratus the green and red rays appear to alternate at each 

 quarter of a circle. Films of selenite are often mounted 

 so as to revolve between the polarizer and the stage. 

 Barker's selenite stage is sometimes used for this purpose 

 i^. 24). With such a stage a set of selenites is usually 



Fis. 24. 



Barker's Selenite Stage. 



supplied, giving the blue, purple, and red, with their com- 

 plementary colors, orange, yellow, and green. By this 

 combination all the colors of the spectrum may be ob- 

 tained. The selenite disks generally have engraved on 

 them the amount of retardation of the undulations of 

 white light, thus : J, f , and \. If these are placed so 

 that their positive axes (marked PA) coincide, they give 

 the sum of their combined retardations. 



The Microspectroscope.* Ordinary spectrum analysis, by 

 determining the number and position of certain narrow 

 lines in the spectra of luminous bodies, called Fraunhofer's 



