SMITH'S INVERTED MICROSCOPE. 711 



stance, in examining fresh Desmidice&ndDiatomacece, the following 

 plan is resorted to, namely : to use a cell, made of a thin piece 

 of brass or glass, perforated with a hole, about a half an inch in 

 diameter ; it is best to give the hole a considerable bevel in one 

 direction, as it facilitates the cleaning of it. Over the small end 



FIG. 432. 



of the hole a piece of thin glass is stuck, with balsam or other 

 cement. "When used, the object to be examined is placed within, 

 and a cover of thin glass placed above. When brass is used to 

 make the cell, it may be as thin as the twentieth of an inch. 

 Dr. Smith remarks that for all observation with high powers, 

 the Inverted Microscope is decidedly superior to the ordinary 

 forms of mounting, for in the latter case, when an object-glass 

 of a iVth or iVth inch focus is used, the focus is too short to admit 

 of the use of cells ; whereas, in the inverted form, as the object 

 is looked at from beneath, the cell may be as thick as one pleases. 

 Another thing connected with this class of observations, is that 

 the Diatomacese and Desmidise can be observed to much greater 

 advantage from beneath than from above, for reasons that will 

 be obvious to persons accustomed to observe these classes of 

 objects. 



Another advantage possessed by this instrument calculated to 

 extend its use for general purposes, is its great capacity for every 

 variety of illumination, without sacrificing the ease and freedom 



