238 USE OF THE MICROSCOPE. 



attached be turned round so that it occupies the position 

 represented by the figure, the ray, r s, will be reflected to E ; 

 if the tube be again turned slowly round, the light will be 

 found to pass through the plate when it has arrived at a 

 distance of 90° from the starting point ; if the tube be turned 

 again, the light will get more and more faint until another 90° 

 are arrived at, when the ray will undergo total reflection ; and 

 so will the changes take place at every quadrant until the 

 starting point is again reached, the ray, r s, being alternately 

 reflected and transmitted. For the purpose of polarizing light, 

 various substances have, from 'time to time, been employed ; 

 amongst the most useful for the microscope wiU be found 

 either glass, blackened on one side, or a bundle of thin glass 

 plates, a crystal of Iceland spar, or a crystalline mineral 

 termed tourmaline. It would be foreign to our purpose here 

 to enter into any of the numerous theories that have been 

 broached, to accoimt for the above described phenomena; for 

 these the reader is referred to the works that are specially 

 devoted to the subject ; but as one of the principal objects of 

 this treatise is to teach those who are uninitiated in micro- 

 scopic science, the use of the various kinds of apparatus sup- 

 pHed with the best achromatic microscopes, we will only take 

 notice of those polarizing instruments which, when applied to 

 the microscope, have been found necessary, in order to aid the 

 observer in his investigations into the structure of organic and 

 inorganic substances. 



The polarizing apparatus most useful to the microscopist 

 has been already described at page 121 ; it consists, as shown 

 in figs. 67 and 68, of two prisms of calcareous spar, constructed 

 after the plan of Mr. Nicol, of Edinburgh, who employed for 

 the purpose a rhomb of the spar divided into two equal por- 

 tions, in a plane passing through the acute lateral angles, and 

 nearly touching the obtuse solid angles. The cut surfaces, 

 having been carefully polished, were then cemented together 

 with Canada balsam, so as to form a rhomb of nearly the same 

 size and shape as it was before the cutting ; by this arrange- 

 ment only one of the two rays into which a beam of ordinary 

 light passing through a rhomb of this spar would be separated 



