S4 The Microscope. 



thin la3'er of fresh cement to a cell already hard and dry, some 

 of them being entirely imaginary. Yet there is one valid 

 criticism to be made, and one result to be avoided. This is 

 that unless the cover is applied at the first trial at the exact 

 point on which it is to remain, the fresh cement will be smeared 

 over the lower surface and so mar the appearance of the mount, 

 and often its usefulness as well. The cover will surely be soiled 

 if it is moved ever so slightly on the fresh cement. To avoid 

 all danger of this kind it is better to use an old and dry cell 

 without the addition of the live cement, however thin it may be. 

 To do this and i)retty surely prevent the entrance of the cement 

 into the cell, after the thin glass has been added, an application 

 of gum arable mucilage may be made around the edge, and when 

 entirely dry and firm, but long before it begins to crack, as it 

 will speedily do, the Brunswick black to finish the mount may 

 be confidently added, as the hardened mucilage will prevent its 

 running under. Should the mucilage itself have a tendency to 

 penetrate into the cell, it will not be so obnoxious in appearance 

 and eflTect as would the black cement. But if it is used in a thick 

 solution, or is applied in a thinner one in several layers put on 

 very carefully, this objectionable feature will not become promi- 

 nent. To accomplish the same object some microscopists add a 

 layer of paraffin before applying the finishing black cement. 

 The paraflfln must be used while it remains fluid after being 

 heated. It may be applied with a hot wire. It hardens as soon 

 as it touches the glass ; yet mounts may be made with Bruns- 

 wick black and without the application of any protecting medium 

 beneath it. But this demands care. 



editor;s 



DEPARTMENT 



"^^S 



Microscopists that take an interest in the construction of the 

 objectives they use, and what microscopists do not, especially 

 if they employ the best at their command, will surely read with 

 a sense of satisfaction mingled with regret and with a certain 



