48 



THE ART OF MAKING MICROSCOPE SLIDES 



Finishing 



required size, moisten the underside by 

 breathing on it, or by smearing on it a 

 little 50% glycerol, and then lay this shde 

 on top of the domed drop of jelly. The 

 cylinder of aluminum is then taken, 

 warmed in the flame until it is somewhat 

 above the melting point of the jelly, and 

 pressed gently on top of the center of the 

 slide until enough jelly has been melted to 

 permit the cover to drop down to the re- 

 quired level. Very little practice is re- 

 quired before one can flatten down the 

 coverslip without in any way disturbing 

 the arrangement of the object at the 

 bottom, which remains throughout this 

 whole process in a layer of solid jelly. In 

 this way the coverslip is flattened without 

 disturbing the object, and one avoids the 

 constant nightmare of endeavoring to 

 lower a coverslip onto rapidly cooling jelly 

 without disturbing the contained object. 

 If the specimens are so thin that pressure 

 may be applied, a clip is attached and the 

 slide placed for a few minutes on a hot 

 table, to permit an equalization of the 

 pressures between the object and the sur- 

 rounding jelly. 



The method of mounting in deep cells is 

 practically identical and is, with the de- 

 scribed tool, just as convenient. The slide, 

 with the cell cemented to it, is warmed on 

 a water bath and then filled with molten 

 glycerol jelly. Sufficient glycerol jelly 

 should be used to leave a high domed layer 

 protruding above the cell; air bubbles 

 must be displaced with a hot needle. The 

 object is then placed in the glycerol jelh', 

 arranged in the required position within 

 the cell, and ^then gently' laid on one side 

 to cool. A moistened coverslip is then 

 placed in the center of the dome and 

 pressed down with the warmed tool until 



it is flattened against the top of the cell. 

 Large cells of this nature should always 

 be placed on a hot plate for at least two 

 or three hours before they are finally 

 cooled, for the commonest cause of break- 

 down of deep jelly mounts is failure to 

 permit the osmotic tension to equahze be- 

 tween the object and its surrounding 

 medium before coohng. 



Finishing Glycerol Jelly Mounts 



Most glycerol jelHes contain so much 

 glycerol that they cannot be left unsealed. 

 They may appear excellent for a few days 

 but sooner or later the glycerol will spread 

 out over the surface of the shde, even to 

 the extent of moistening the label and 

 causing it to fall off. The slide, however, 

 must be cleaned before it can be sealed. 

 Make sure that the slide is cold — a refrig- 

 erator is very convenient — and then with 

 a sharp razor blade or scalpel trim away 

 all the unwanted jelly. Then remove the 

 smears that are left with a moist cloth 

 and use another cloth moistened with 95 % 

 alcohol to remove any glycerol which may 

 remain on the glass. There is always some 

 residual gh'cerol, however, which makes it 

 essential that the first coat of cement 

 should be a gelatin-dichromate mixture of 

 the type of Riiyter 1934 (Chapter 28, 

 V 12.1). This is melted on the water bath 

 and applied to the edges of the coverslip 

 with a brush. The slide need not be 

 warmed since these cements stay molten 

 at quite low temperatures. The slide is 

 placed on one side to dry and then given 

 an additional coat of any waterproof ce- 

 ment. It is a worthwhile precaution, be- 

 fore applying the last coat of cement, to 

 wipe off the slide with 95% alcohol to re- 

 move any trace of glycerol. 



Specific Example 



Preparation of Small Crustaceans in Glycerol Jelly 



The term small crustaceans, as here 

 used, includes all specimens up to the size 

 of a Gammarus, as well as the numerous 

 larvae which are found both in fresh and 

 salt waters. This group contains a large 

 number of fascinating forms of universal 

 distribution. A brief word must be said on 



methods of collection and preservation be- 

 fore passing to actual mounting. 



Free-swimming crustaceans, whether 

 marine or fresh-water, are collected by 

 means of a plankton net. This is a long 

 conical net, made by the professional from 

 bolting silk, and by the amateur from the 



