118 MICROSCOPIC ILLUSTRATIONS. 



light, of inestimable value in particular researches, and 

 with particular microscopes. 



" But, independent of these advantages, the method of 

 using a lateral pencil, m, has the great advantage of not 

 requiring any thickness in the rhomb. A Nicol's prism, 

 and a rhomb in which the two principal images, b, c, are 

 used, must be about an inch thick, in order to be effica- 

 cious ; but the distances, m, n, or m, b, are the same at 

 all thicknesses, so that Ave can use rhombs for this pur- 

 pose which are quite useless for any other. 



u It is scarcely necessary to add, that similar rhombs in 

 which either the principal images, b, c, or the lateral ones, 

 m, n, are used, may be employed for the analyzer. For 

 this purpose, a thin plate, in which m, or n, is white, is 

 peculiarly applicable, as it enables us to see at once the 

 whole field of the microscope*." 



In all the effects produced by the foregoing arrange- 

 ments, the field of view is either black or white. By 

 employing, however, the stage represented by fig. 25, 

 and inserting between the upper plate and the circular 

 one a plate of crystal or unannealed glass, the back 

 ground may, at pleasure, be made to assume a variety of 

 colours, so that the crystals shall appear like so many 

 coloured gems set in different tinted mountings ; whilst, 

 without this addition, they all seem to be set on black 

 velvet. 



Considerable advantage is derived, also, from my con- 

 trivance of the double stage, in the examination of Mr. 



* Treatise on Microscopes, p. 104. 



