ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 723 



the cover or slip I dip a small needle in the solution and wipe it once 

 flatwise across the glass. 



There are many little wrinkles which the worker will acquire 

 from time to time. One of the most important of these is the art of 

 using the finger as a lever for moving diatoms or other objects into 

 position when very slight movements are necessary. To do this, 

 move the slide by hand until the point of the finger is just behind the 

 object to be moved ; then, by racking down with the coarse adjust- 

 ment, the glass point pushes the object ahead of it. By a succession 

 of pushes the object may be moved into any desired position. The 

 coarse adjustment may be used in a similar manner for turning diatoms 

 on edge or upside down, by pushing them against some fixed object 

 and forcing the glass point under them. By using a point rather 

 firmer than usual, the valves of a diatom may be separated. To do 

 this I usually fasten the diatom on a slide which has been coated 

 with gelatine, and when it is firmly fixed, the upper valve may be 

 punched off without much difficulty. 



Another wrinkle, and quite a valuable one too, is what might be 

 called a scientific use of the imagination. Many cannot work a 

 mechanical finger well without an erecting eye-piece, on account of 

 all movements appearing to be reversed. This difficulty will disappear 

 if the worker will just imagine, as he holds the stage and moves it, 

 that he is holding the finger and moving it ; all motions will then 

 appear to be perfectly natural. I might state here that a mechanical 

 stage is not the best for this kind of work. 



There is a popular misconception in regard to the mechanical 

 finger which it may not be amiss to mention. Many regard it as a 

 kind of scientific plaything — an instrument used merely for arranging 

 diatoms so as to form pretty slides. I have no doubt but that it will 

 come eventually to be regarded as one of the microscopist's most 

 valuable accessories, and one which every worker will require. It 

 may be used not only in handling and studying diatoms, but also 

 other objects which are too small to be handled in the ordinary way. 

 In studying the Infusoria, for instance, a drop of water containing 

 them may be placed in a concave slide, then, when the water has been 

 almost evaporated, or has been removed by means of bibulous paper, 

 the Infusoria may be picked out with the mechanical finger and 

 studied, or deposited on a slip for mounting. A firm thread of dark- 

 coloured glass is best for this. 



In studying diatoms, a mechanical finger is almost indispensable, 

 for it may safely be said that one is not thoroughly acquainted with a 

 diatom until he has turned it over and viewed it in all its aspects. In 

 mounting diatoms for study it is well to mount a number of the same 

 kind in various positions, so as to display the various spines, undula- 

 tions, or other peculiarities. How often it happens, too, that in a 

 mixed gathering of diatoms — and it is not easy to obtain pure 

 gatherings — we find a rare frustule which we should like to preserve. 

 By means of a mechanical finger the frustule may at once be selected 

 and mounted. 



When one wishes to arrange diatoms so as to form symmetrical 



