ON THK POLARIZATION OF LIGHT. 239 



is transmitted, the other being rendered too divergent ; hence 

 it has received the name of the single image prism. Two of 

 these must be provided, one to be adapted to the under 

 surface of the stage, and termed the polarizer, as shown by 

 fig. 66, whilst the other, called the analyzer, is placed above 

 the eye-glass. To effect the same purpose, some persons 

 employ a bundle of glass plates as a polarizer, and a tourma- 

 line as an analyzer ; but the colour of the latter is often ob- 

 jectionable in the compound microscope, M'here everything 

 that win diminish the brightness of the light and brilliancy of 

 the coloiu-s should be avoided. It will be found that a tour- 

 maline of a neutral tint forms an excellent analyzer, having 

 one great advantage, viz., that when placed over the eye- 

 piece, the field of view is not contracted as it is when a Nicol's 

 prism is employed. " The best tourmaline to choose," says 

 Mr. Woodward,* " is the one that stops the most light when 

 its axis is at right angles to that of the polarizer, and yet 

 admits the most when in the same plane." 



In order to illustrate some of the most striking phenomena 

 of polarized light in a very simple way, by the achromatic 

 compound microscope, the apparatus consisting of two prisms 

 and a film of selenite, as described at page 121, will be nearly 

 all that will be required as well as for the examination of 

 minute animal, vegetable, and mineral structures; we will, 

 therefore, proceed at once to the application of the same. 



Method of using the Polarizing Apparatus. — The polarizing 

 prism represented by figs. 66, 67, and 68, having been adapted 

 to the under surface of the stage, either another prism or a 

 plate of tourmaline is to be placed over the eye-piece, or in 

 the end of the draw-tube, as shown by fig. 75, and the light 

 having been reflected through them by the mirror, the step 

 that next becomes necessary is to make the axes of the two 

 prisms coincide : this is done by revolving either the upper or 

 lower one, and noticing whether, in some positions, the light 

 is completely cut oflT, and in others wholly transmitted ; if the 

 field of view be not darkened twice during one revolution of 



* Familiar Intrnditction to the Slndy nf Polarized Light. Second 

 Edition. Page 3-5. 



