20 The Microscope. 



excellent lenses. ' It is only the tests which are usually so 

 mounted. 



If the microscopist uses square covers for his mounts, as he 

 may, he must of necessity make square cells. A circular cover 

 will not consent to be applied to a square cell, and a circular 

 cell will not act well with a square cover glass. It is here a 

 case of like to like so far as form is concerned. But to make a 

 square cell a pattern is necessar3^ This may be easily made by 

 drawing with ink a square on card-board and placing the slip above 

 it. The square cell may then be prepared b}' the application of 

 the brushful of the semi-fluid cement, and set aside to dr3\ If 

 the worker's hand be stead}^ the result will be satisfaetor3' ; 

 otherwise it will have a very shak}- appearance. If one applica- 

 tion of the cement does not make the cell deep enough, as it 

 probablj' will not, other layers maybe added after those beneath 

 are dr}^, or nearly so. In this way, by the application of suc- 

 cessive coats, a cell of almost any depth may be built up. This 

 method is also applicable to circular cells. The\' too, ma}^, be 

 made as deep as is desired by adding layer after layer, although 

 it demands some skill to do this cell building by free hand and 

 have the result satisfactor3\ There are consequently other and 

 better methods of making deep cells. Unless the cement is of 

 extraordinary tenacity and firmness, these deep rings are apt to 

 be disappointing, because the}^ will leak in time, and often in a 

 short time, letting the air in and the medium out; the slide is 

 then a sad thing to contemplate. For deep cells to hold very 

 thick objects it is better to use a method to be referred to here^ 

 after; but for fastening down the cover and for making shallow 

 cells, cement is always used, and if it is good, nothing more is 

 needed. 



Some microscopists are fond of tr3'ing every new cement rec- 

 ommended in the books and in the journals. A new cement 

 ma}^ seem to be a success in the hands of the inventor who is m 

 such a hurry to recommend it to the world of microscopists that 

 he does not give his invention a chance to show its q la.ities; 

 but because it stands the wear and tear of a short time h.3 con- 

 cludes that it is a fine thing, and gives it to others ; and some 

 of us are disposed to try every novelty of the kind that comes 

 out. This is not to be commended. It is far better to select 

 some one cement, or perhaps two, which have been thoroughly 



