178 MICROSCOPIC OBJECTS. 



of a plate of tourmaline fitted to a cap which covers over 

 the eye-piece. The lower figure is the polarizer, which 



consists of a single-image prism : its mounting fits into 

 the large aperture in the stage g. In using this appa- 

 ratus first insert the polarizer in the stage, then place 

 your object upon it, and direct a strong pencil of hght 

 through them ; now adjust your microscope to distinct 

 vision ; and lastly, cover the eye-piece with the analyser. 

 By turning the analyser until the whole of the hght is 

 stopped except that which is depolarized by the object, 

 the crystals appear hke gems upon black velvet : when 

 this is effected, if the analyser be turned a quarter round, 

 the crystals will assume complementary colours, and the 

 ground will then be luminous. For viewing thin sections 

 of the hoofs of animals, quOls, bone, teeth, the palate of 

 the whilk, &c., a thin plate of selenite should be placed 

 under them. 



In viewing transparent objects it is often desirable not 

 to permit any. hght to fall upon the objects ; otherwise a 

 fog is produced. This may be prevented by fitting the 

 shade (represented in the annexed engraving) upon the 

 stage by means of its two pins. 



The following is a table of magnifying powers of Mr. 

 Pritchard's Vertical Achromatic Microscope. They are 

 calculated by a ten-inch standard of sight, and are given 

 in diameters. Note. — Those who wish to ascertain 



