294 PRACTICAL MICROSCOPY. 



then sought for, such as diatoms, foraminifera, or other 

 minute specimens, and brought into the centre of the field; 

 the point of the bristle is then lowered by the screw until 

 it touches the desired object, which usually adheres to it at 

 once. 



FIG. 240. 



Some operators recommend a cat's whisker for the pur- 

 pose of using with the mechanical finger, and no doubt it is 

 as good as anything. Still the worker with this piece of 

 apparatus will find that often, especially when working 

 with ordinary bristles, it is almost impossible to deposit the 

 object exactly where it is required, so pertinaciously does it 

 adhere to the bristle. Prof. H. L. Smith has advised the 

 use of a slender thread of glass for this purpose, but taking 

 all things into consideration the cat's whisker seems to be 

 the best. 



The diatoms are picked one by one and placed where 

 desired. To prepare the slide, put it on a turntable, and 

 with a pen make a small circle in the centre to guide in 

 placing. On the reverse of the slide put a tiny drop of 

 pure distilled water, with a small fragment of gelatine in 

 it ; so that when the diatoms are arranged, breathing on 

 them will bind them into the size. Foraminifera, Poly- 

 cistina, and other similar objects may be arranged in the 

 same manner. 



As another instance of dry preparation may be given a 

 method for mounting starch granules. The starch, say 



