OP THE MICROSCOPE. 229 



to the objective are interfered with to a considerable extent. 

 The objections to mounting opaque objects in cells with mov- 

 able covers are that even during the short period in which they 

 are exposed for examination they are liable to be contaminated 

 with dust, the cover is liable to be lost, and the object when 

 exposed is liable to mechanical injury. Therefore, while a few 

 brilliant and striking objects, such as minerals, seeds, etc., may 

 well be mounted in open cells, all delicate and valuable objects 

 should be permanently covered. 



Of late years the most popular cell for opaque objects is un- 

 doubtedly that devised by Prof. Hamilton L. Smith, and known 

 as the wax cell. Various methods of making it are in use, the 

 following being that originally published by the inventor: 

 Take a circular disc of thin sheet wax, which is easily cut with 

 a common gun punch from the sheet wax ordinarily used for 

 making flowers, and attach it by means of heat to the cen- 

 tre of a glass slide. A brass curtain ring, of which the inte- 

 rior is the same size as the disc, is then slightly warmed and 

 laid on the wax; to which it, of course, adheres. The object is 

 fixed to the wax by slightly moistening the surface of the latter 

 by a minute drop of turpentine. When dry, a cover, which 

 exactly fits into the bevel of the ring is attached thereto with a 

 little cement, and the whole may then be finished off on the 

 turn-table. 



The appearance of objects mounted in this way is very ele- 

 gant, and consequently it has become a general favorite. It 

 has, however, recently been condemned in most unsparing 

 terms by the inventor, who has found that the under surface 

 of the cover becomes in time coated with a deposit which 

 causes a glare that entirely prevents clear vision. As soon as 

 this difficulty was announced, an animated correspondence 

 took place in the scientific journals, and it was found that 

 while some microscopists had experienced this difficulty, it 

 had never occurred to others. The final conclusion seems to 

 be that the difficulty arises from the kind of wax used, and the 

 method of attaching the object to it. The wax should have 

 been thoroughly melted at a temperature as high as it will 

 bear, so as to drive off all volatile matter, and instead of using 

 discs punched from sheets, the wax should be applied in a 



