64 PRACTICAL TREATISE ON THE MICROSCOPE. 



a large firm stage for supporting the objects under exami- 

 nation; and as it is found that, after a little practice, 

 an object can be moved about on the stage with very great 

 nicety, the stage movements may be dispensed with where 

 low powers only are employed ; but with doublets and trip- 

 lets some more delicate adjustment than that of the hand 

 becomes necessary, and such an instrument as that described 

 by fig. 38 should be had recourse to, where both fine and 

 coarse movements for the magnifiers are proAdded, and all 

 parts of the object can be carried under the lens by the ad- 

 justable stage. 



The magnifying powers generally employed with single 

 microscopes, may be divided into those consisting of one lens 

 only, and those of two or three lenses combined, from which 

 circumstance they are termed doublets or triplets. In the 

 first class are included all the powers, from two inches up to 

 one quarter, and sometimes one-tenth of an inch; these 

 should be set in flat cells, hke that seen in fig. 31, and be 

 made to drop easily into the lens-holder. Some persons use 

 planoconvex lenses for the very low powers, in these the 

 centre of the field will be perfect and well defined, but the 

 margins not so ; hence, both theoretically and practically, it 

 wiU be found that double convex lenses are the best 

 for low powers, especially for dissecting; those who are in 

 possession of a two-inch achromatic object-glass, wiU soon 

 learn that where very careful work is required, a glass of that 

 description will be by far the most pleasant to use. Mr. 

 Powell supplies, with his dissecting microscope, represented 

 by fig. 31, sometimes as many as seven lenses, the four lowest 

 range in focal length from two inches to half-an-inch, and the 

 fifth, is a Coddington lens, of a quarter of an inch focus, the 

 remaining two being doublets, one of one-tenth, the other of 

 one-twentieth of an inch focus. Two of these largest lenses 

 are double convex, the other two either crossed or plano- 

 convex. After the ordinary lens of half-an-inch in focus, the 

 next increase in the magnifying power should be supplied by 

 the Coddington lens, represented by figs. 25 and 26; this 

 affords a large field, equally good in all directions, and its 



