208 MOUNTING OBJECTS DRY. 



the penetration of air or fluid much less, if the immediate margin 

 of glue be left both outside and inside the cell. 



154. Mounting Objects Dry. — There are certain objects which, 

 even when they are to be viewed by transmitted light, are more 

 advantageously seen when simply laid on glass, than when they 

 are immersed either in fluid or in balsam. This is the case espe- 

 cially with sections of bones and teeth, much of whose internal 

 structure is obliterated by the penetration of fluid ; and also with 

 the scales of Lepidopterous and other Insects, whose minute sur- 

 face-markings are far more distinct when thus examined, than 

 when treated in any other way. For preserving such objects, it is 

 of course desirable that they should be protected by a cover ; and 

 this must be so attached to the glass slide as to keep the oliject in 

 place, besides being itself secured. For this purpose, Sealing-wax 

 varnish is often used, but it is unsuitable on account of its brittle- 

 ness when dry ; Brunswick Black or Gold-Size mixed with Lamp- 

 black is much to be preferred, and, if carefully laid on, will not 

 tend to run in between the cover and the slide. If the object have 

 any tendency to curl up, or to keep off the cover from the slide by 

 its own ' spring,' it will be useful, while applying the varnish, to 

 make use of pressure, such as that afforded by the Spring-Clip* 

 represented in Fig. 97, and this pressure should not be remitted 



until the varnish is dry enough 

 F lG - 9 ^- to hold down the cover by itself. 



"Where the object is thin, and 

 not liable to be injured by a 

 gentle heat, the best method is 

 to use a Cement-cell (§ 166) 

 thoroughly hardened, and after 

 the object has been placed in it, 

 and its cover laid on, the slide is 

 warm edf sufficiently to soften the 

 Spring-Clip. r i n g of Cement, on which the 



'cover is then carefully pressed 

 down, so as at the same time to attach itself and to fix the object. For 

 mounting delicate objects, the thinner slides should be selected ; 

 and for very difficult Test-objects, it is advantageous to employ thin 

 glass below as well as above the specimens, for the sake of diminish- 

 ing the aberration which the illuminating pencil sustains in its 

 passage to the object, and for allowing the Achromatic Condenser 

 to approach the object as closely as possible. For this purpose the 

 simplest method is to take a slip of "Wood (preferably either maho- 

 gany or cedar) of the ordinary size of the glass slide (3 in. by 1 in.), 



* This very useful little implement is an improvement by Mr. Jabez 

 Hogg upon a form originally devised by Dr. Maddox. It is sold at a 

 very cheap rate by Messrs. Baker, Mr. Collins, and other dealers hi 

 Microscopic Apparatus. 



