122 



PRACTICAL TREATISE ON THE MICROSCOPE. 



seen that this crystal, in common with many others, has the 

 property of bending the rays of light that have traversed the 

 polarizer, and of causing them to pass through the analyzer ; 

 according to the thickness of the crystalline plate, so wUl 

 either a green or red colour prevail. The cause of these 

 appearances, and the various applications of the polarizing 

 apparatus, will be further aUuded to in the chapter devoted 

 to this subject. Some microscopists employ a bundle of thin 

 glass plates for a polarizer, and a tourmaline for an analyzer ; 

 but the colour of the latter renders its use objectionable. 

 Condensing Lens. — An indispensable instrument for the 



illumination of opaque ob- 

 jects, or of the mirror when a 

 great quantity of light is re- 

 quired, is the condensing lens 

 or bull's-eye. This is gene- 

 rally a planoconvex lens of 

 great thickness, from two to 

 three or more inches in di- 

 ameter, mounted in the man- 

 ner represented by fig. 69, on 

 a stem of brass attached to a 

 heavy circular foot. Upon 

 this stem a short tube, hav- 

 ing another piece of simi- 

 lar tube fastened into it at 

 right angles, is made to slide ; 

 into this last fits a short rod 

 or tube, to support the lens 

 and allow of its being in- 

 clined at any angle. This 

 method of mounting the lens 

 is adopted by Messrs. Koss 

 and Powell; but Mr. Smith, 

 following Mr. Tulley, em- 

 ploys the same land of stem 

 and foot, and, in addition 

 to being inchned at any angle, 

 the lens is provided with 



Fig. 69. 



Fig. 70. 



