DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 115 



with as intense a light as can be made to pass through 

 them. Now, insert into the socket under the stage, at 

 e, a single-image prism, and then examine the crystals. 

 They will still retain their usual colour, if any, though 

 the light passed through them is polarized. This prism is 

 termed the polarizer. 



Next, unscrew the cap of the eye- piece, P, and apply 

 another cap, containing a tourmaline, or single-image 

 prism. This latter is shewn separately at P'. By 

 turning one of the prisms gently round, it will be 

 noticed that in certain positions no more light will be 

 transmitted than just so much as passes directly through 

 the crystal ; whilst the crystal, which, before the upper 

 prism or analyzer was added, was colourless, now dis- 

 plays the most brilliant colours. If a crystal be se- 

 lected exhibiting a deep orange tint, and the eye be fixed 

 upon it as the prism is turned a quarter round, the 

 colour will gradually change to green, and the light 

 will, at the same time, be freely transmitted through the 

 microscope ; in other words, the field of view will be- 

 come illuminated in the same manner as when a trans- 

 parent object is viewed in the ordinary way. Continue 

 turning the prism another quarter of a revolution, and 

 the light will again be stopped, and the crystal will 

 resume its rich orange tint. The two prisms are mounted 

 in brass tubes, with caps. The longer one is to be used 

 under the stage, as polarizer, and the shorter above, as 

 analyzer. 



When the analyzer is a plate of tourmaline, instead of 



