D. B. WARD, M. D. 83 



and water from the vial in which they nre kept and sub- 

 stitute an equal quantity of distilled water. A few 

 drops of the mixture are then placed on a thin cover- 

 glass and evaporated with very gentle heat. Then the 

 cover is mounted on a glass slip (3x1 inches) in Canada 

 balsam, or liquid amber, or styrax, and we have a per- 

 manent spread slide which can be examined at any time. 

 Of course in a preparation like this the diatoms are all 

 mixed up, many are broken, and any particular speci- 

 men difficult to lind, and some years ago microscopists 

 undertook to pick out and mount different s^Decies sepa- 

 rately. If you put a fine hair in the end of a stick for 

 a handle, and then try to pick up under the microscope 

 a minute object, the chances are a thousand to one that 

 you won't succeed for a long time, but if by constant 

 practice you do succeed in picking uj) a diatom for in- 

 stance, you will require a longer training still to make it 

 let go of the hair when you want to put it down anywhere. 

 It will illustrate better than anything I know the utter 

 and total depravity of inanimate things. A little instru- 

 ment was devised by Mr. Zentmeyer, of Philadelphia — 

 a famous maker of microscopes in his day — which sim- 

 plifies things somewhat. It is called a "mechanical 

 finger," and is fastened to the nose-piece of the micro- 

 scope. The hair fastened into the end of the iron rod is 

 raised or lowered by means of a screw and the motion in 

 the horizontal plane is attained by shoving the slide 

 carrier about. This obviates to a great extent tl>e effects 

 of. tremor in the hands which exists in all persons to a 

 greater or less degree, yet even with its help a good deal 

 of practice is required before good results can be ob- 

 tained ; and as the instrument must be vertical and as 

 the operator must adapt his own position to that of the 

 microscope, he needs above all things, except perhaps un- 

 limited patience, a good strong back, as the attitude 

 necessarily assumed is a very trying one. 



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