ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



207 



are partially extended the tube is of the standard 160 mm. length ; the 

 binocular Microscope is thus rendered as compact and serviceable as the 

 monocular type. In the older types of binocular Microscopes a tube of 

 about 9 in. to 10 in. in length was required in order to extend the eye-pieces 

 to the necessary interocular distance ; but examination of the diagram 

 fig. 23 shows that, owing to the peculiar construction of the prism, the 

 tubes, instead of converging towards the prism, converge to an apex 

 about 3ij in. below it ; thus, although the standard angle of normal con- 



', 



Fir;. 23. 



vergence is retained, the tubes need not be long to give the required 

 separation for the eyes. The tubes converge at an angle of about 1 I . 

 This will be found in practice to give absolute comfort for either long or 

 short periods of working. The eyes are in exactly the condition re- 

 quired for reading a book. Any make of object-glass or eye-piece of 

 the standard size can be used. There are absolutely no special require- 

 ments — a revolving nose-piece, an objective changer, or any form of 



P 2 



