218 THE MICROSCOPE. 



examined under the microscope with an inch object- 

 glass, and the siliceous valves of the diatoms picked 

 out with a coarse hair or bristle. 



Dr. Rezner's Mechanical Finger (fig. 143/fc) for select- 

 ing and arranging diatoms, adaptable to any micro- 

 scope, is made to slip on to the objective far enough to 

 have a firm bearing, and so that the bristle point can 

 be brought into focus when depressed to its limit. It 

 is clamped in its place by a small thumb-screw. The 

 bristle holder slides into its place so that it can be 

 brought into the centre of the field. When using the 

 finger, the bristle is first raised by means of the micro- 

 meter screw till so far within focus as to be nearly or 

 quite invisible, then the objective is focussed on to the 



FIG. 1437t. Rentier's Mechanical Finger. 



slide, and the desired object sought for and brought 

 into the centre of the field ; the bristle point is then 

 lowered by the screw until it reaches the object, which 

 usually adheres to it at once, and can then be exam- 

 ined by rotating the bristle wire by means of the 

 milled head. Professor H. L. Smith offers some useful 

 hints which will facilitate the use of this finger. 1 



The medium used for mounting diatomaceos is of 

 very considerable importance, inasmuch as their visi' 

 bility is either diminished or much increased there- 

 by. Professor Abbe, experimenting with the more 

 minute test-objects, diatoms, &c., found monobro- 

 mide of naphthaline gave increased definition to most 

 of them. This liquid is colourless, somewhat of an 



Journal of the Royal Microscopical Society, Vol. II., 1879, pp. 952-3. 



