220 THE MICROSCOPE. 



taken to have the specimens quite dry before soaking them 

 in turpentine. Objects mounted in cells, on the contrary 

 should have become perfectly saturated with the mounting 

 fluid before being finally secured. 



It is preferable to mount and preserve specimens of animal 

 tissues in shallow cells, to avoid undue pressure on the 

 preparation. Cells intended to contain prejDarations im- 

 mersed in fluid must be made of a substance impervious 



to the fluid used; on the whole, the 

 most useful are those made with 

 circles of thin glass, cemented to 

 the glass-slide with marine glue, 

 such as we have here represented 

 (fig. 140). The surface of the glass 



Fig " U0 Mu!SZ't lS f ° r should be sli g htl y roughened before 



applying the cement. 



Different modes of mounting may be employed with 

 advantage, to show different structures ; entomological 

 specimens, such as legs, wings, spiracles, tracheae, ovi- 

 positors, stings, tongues, palates, corneae, &c. show best 

 in balsam : the trachea of the house-cricket, however 

 should be mounted dry. Sections of bone show bes*, 

 when mounted dry, or in a cell with fluid. Scales of 

 butterflies, moths, &c. should be mounted dry. Other 

 objects, as sections of wood and stones of fruit, exhibit 

 their structure best in a cell with fluid. 



There are some objects much more difficult to prepare 

 than others, and which tax the patience of the beginner 

 in a manner which can hardly be imagined by any one 

 who has never made the attempt. The structure of many 

 creatures is so delicate, as to require the very greatest care to 

 prevent mutilation, and consequent spoliation of the spe- 

 cimen. The beginner, therefore, must not be discouraged 

 by a few failures in commencing, but should persevere 

 in his attempts, and constant practice will soon teach him 

 the best way of managing intricate and difficult objects. 

 The room in which he operates should be free from dust, 

 smoke, and intrusion, and everything used should be kept 

 scrupulously clean, since a very small speck of dirt, which 

 may be almost invisible by the naked eye, will assume 

 unpleasant proportions under the microscope, and not only 



