250 



USE OF THE MICROSCOPE. 



It consists of a plate of brass, four inches long, an inch-and a- 

 half broad, and one-fifth of an inch thick, having a piece of 

 raised brass screwed to it, against which objects may rest 

 when the microscope is inclined; in the centre of the brass 

 plate there is a hole, one inch in diameter, into which is fitted 

 a ring of the same metal, with a shoulder on its under side to 

 receive certain "cells, into which plates of selenite are fitted ; 

 this ring is capable of being revolved either to the right or 

 the left of a central index or dart, by means of an endless 

 screw, turned by the small handle seen on the right side of the 



Fig. 158. 



figure. P A i, P A |, P A i, represent three brass cells, into 

 each of which are burnished two plates of thin glass, having 

 between them films of selenite of difierent thicknesses. The 

 dart at P A denotes the direction of the positive axis of the 

 selenite, and the figures -i, |, &c., denote the parts of a vibra- 

 tion retarded by each disc, which, by their super-position and 

 variation in position, by means of the endless screw motion, 

 produce all the colours of the spectrum. 



Advantages of Polarized Light to the Microscopist. — The 

 application of this modification of light to the illumination of 

 very minute structures has not yet been fuUy carried out, but 

 stiU there is no test of differences in density between any two 

 or more parts of the same substance that can at all approach 

 it in delicacy. All structures, therefore, belonging either to 

 the animal, vegetable, or mineral kingdom, in which the 

 power of unequal or double refraction is suspected to be 

 present, are those that should especially be investigated by 



