98 MICROSCOPIC ILLUSTRATIONS. 



same amplification is obtained with a shallow one, and a 

 deeper object-glass : the preference, therefore, when 

 great accuracy is required, should always be given to the 

 latter arrangement wherever it is admissible. 



An engiscope may be furnished with one or more sets 

 of object-glasses. They all have similar screws for fixing 

 them into the lower end of the body. When a set is 

 composed of only two or three lenses (the usual con- 

 struction of shallow ones), it can give but one magnify- 

 ing power. In like manner, when a set is constructed so 

 as to take up a very large pencil of rays with the utmost 

 definition, although composed of several pairs of lenses, it 

 can give but one perfect power. Such sets should never 

 be separated. In many cases, where a maximum angle 

 of aperture, or a perfect correction, can be dispensed 

 with, one set composed of three pairs of cemented lenses 

 may give by separation three different magnifying powers. 

 These sets are generally made so that their amplifica- 

 tions, in diameters, are in the ratio of one, two, and 

 three ; and are preferred by some observers. 



Achromatic object-glasses for microscopes are of various 

 foci, differing from two inches to 1- 16th of an inch. As a 

 table of all the varieties would occupy more room than its 

 worth would justify my giving, I shall specify only those 

 which are mostly used in the table-microscope. They 

 afford a very wide range of powers, and one or the other of 

 them is suited for every class of objects which can be sub- 

 mitted for examination. It is seldom that an instrument 

 has the whole series attached to it ; and since many per- 



