8 METHODS. 



filtering-paper ; a drop of dilute glycerine is added best with the 

 glass stem of the bottle holding glycerine to the specimen, which 

 is then worked down to the center of a glass slide. Here the 

 specimen is spread out, if necessary, with two needles, its position 

 corrected, and the covering-glass gently placed over the drop, so 

 as to avoid including air-bubbles. With a little practice and 

 skill we learn to add the exact quantity of glycerine. Should 

 the drop prove to be too small, viz., if a corner or edge of the 

 covering-glass wants glycerine, a small droplet is approached to 

 that edge, and will flow under by capillary attraction. If too much 

 glycerine be taken, it must be drained off by moist filtering-paper, 

 and the slide cleansed carefully with a piece of such paper folded 

 up and moistened. The sealing together of both glasses should 

 be accomplished by painting varnish in the shape of a narrow 

 but heavy rim along the edge of the covering-glass ; but great 

 care must be taken to have both slide and cover first absolutely 

 clean and dry. 



The only liquid which can be fully recommended for mount- 

 ing hardened specimens is glycerine in the purest chemical 

 condition, to which distilled water (about one part of water to 

 three parts of glycerine) is added. Mounting in Canada balsam 

 or in damar varnish is objectionable, as the specimens in these 

 liquids in time clear up to such an extent as to become unfit for 

 amplifications of the microscope exceeding 300 or 500 diameters: 

 Long-continued trials, as regards the value of both methods, 

 have led me to this conviction. Specimens of any description, 

 mounted in Canada balsam or in damar varnish, are not suitable 

 as test objects. To-day, the power of definition of a lens should 

 be tested exclusively on living objects, such as infusion organ- 

 isms, fresh blood corpuscles, saliva corpuscles, etc. The process 

 of mounting in glycerine is simpler and easier than any other 

 method, and, if all precautions mentioned are carried out with 

 care, no change of the specimen will take place. True, glycerine 

 specimens need more careful handling than balsam specimens, 

 but their value is decidedly greater than that of the latter. 



In order to make glycerine mounting safe, it is preferable to 

 delay applying the varnish for twenty -four hours, as the surplus 

 water by that time will have evaporated. Should too little 

 glycerine be used, the inclosing varnish will run under the cover 

 and deprive the specimen of its neat appearance; should too 

 much glycerine be left between the two glasses, it often happens 

 that after months or years the glycerine finds its way through 



