866 RECORD OF CURRENT RESEARCHES RELATING TO 



this case it will be advisable to place the object iu the centre of the 

 cell, in a quantity of the medium so small that on j)ressing down the 

 cover the drop will not quite fill the cell, and consequently none will 

 be forced out. The cement may then be run iu under the margin, as 

 above described. If the medium be thin and likely to spread over 

 the floor of the cell before the cover can be applied, it will be better 

 to suspend a small drop from the centre of the cover, and bring it 

 down upon the object ; and in any case the cover should be moistened 

 with the medium before applying it. 



The author adds in a note that he finds Tuckett advises the use 

 of a cover larger than the cell, in order to i^revcnt running in ; b\it 

 as he does not withdraw the fluid from the space round the cell, his 

 method gains no advantage over the ordinary plan in security from 

 leakage. 



Thickness of Cover-glasses. — Dr. C. Eeddots points out,* in 

 answer to a complaint that cover-glasses are not accurately assorted 

 as to thickness by the dealers, for which it was said " there seems 

 no good reason," that the good reason is to be found in the extra 

 cost that would be entailed by measuring, so that the matter is better 

 left to each microscopist to do. Moreover, in these days of objectives 

 with large working distance — homogeneous-immersion and others — 

 there is no such necessity as there used formerly to be to hunt for 

 very thin cover-glasses. 



Finishing Slides. f — A writer in the ' American Monthly Micro- 

 scopical Journal,' having used dammar dissolved in benzole as a 

 mounting medium for some time past, finds that, when thoroughly 

 dry, the gum becomes brittle, and a slight jar is apt to start the 

 covering glass, and rapid destruction of the slide follows. He has 

 found it necessary, therefore, to run a ring of some tough material 

 around the covering glass to protect it, his efforts being directed to 

 discovering a material that would give the necessary strength, that 

 can be easily handled, so as to make a neat finish. The best results 

 can be obtained by the use of a thick copal furniture-varnish — what 

 is known as rubbing-varnish — using the thickest, finest varnish that 

 can be procured, and putting enough dragon's blood in the bottle to 

 give it colour, without destroying its transparency. It should be so 

 thick that a small drop will not flow from the camel's-hair brush. 

 The older it is the better. 



The slide, having been cleaned of superfluous gum or balsam, should 

 have a little shellac varnish run around in the angle formed by the 

 covering glass and the slide to prevent the coloured varnish from 

 running under the cover in the subsequent operations. When this is 

 dry, which will be in a few minutes, the slide is mounted on the turn- 

 table, and a sufficiency of the varnish put round the edge of the 

 co^j^ering glass, extending over the slide. The turntable is then put 

 in rapid revolution, and with the point of a knife applied to the 

 glass, first outside on the slide and afterwards inside on the covering 



* 'Am. Mrm. MiVr. Jmirn.,' i. (1S80) p. 123. f I'-'^-, PP- 122-3. 



