166 MICROSCOPIC OBJECTS^. 



they can be seen without removal, which cannot be done 

 when lying upon their edges. 



A convenient size for a fixed cabinet, where slips of 

 wood and grooved edges are not required, is 12 inches 

 long, 9 inches wide (from front to back), and ^ of an 

 inch deep. These are the inside measures of each 

 drawer, which will contain 36 slides of the standard size, 

 namely 3 inches long by 1 inch wide, or 84 of the second 

 size. A cabinet of 20 shallow drawers of these dimen- 

 sions and four deep ones for opaque objects, wiU form an 

 excellent museum capable of containing nearly 2000 

 illustrations of animal, vegetable and mineral specimens. 

 Such a collection is now before us. 



(17.) Traversing Motion for Objects under the MicrO' 

 scope. — It is often desirable to move the object across 

 the field of view without altering the illumination, 

 especially with very minute ones, and in the case of ani- 

 malcules, Entomostracea, &c. When a motion is to 

 be communicated to the objects, it is generally effected 

 by two screws working in separate plates at right angles 

 to each other. By this means a motion is obtained in 

 any direction by turning first one and then the other, or 

 the same is effected by two racks and pinions or by levers : 

 the latter is the simplest, but unless properly constructed 

 is very hable to become unsteady, and is often in the 

 way, preventing a candle, &c. fi'om approaching close 

 behind the stage ; the disadvantage of the other method is, 

 that the observer is usually compelled to use both hands 

 in this adjustment. The plan contrived by Mr. Pritchard 

 is a single screw, which is made to act at the same instant 



