MOUNTING OF MICROSCOPIC OBJECTS. 31 



We have now a perfectly opaque stop, with a clean glass surface, 

 into which the most delicate object cannot sink and be lost, as is 

 always the case if mounted directly upon the surface of asphalte, 

 without the intersection of the thin glass. Bear this carefully in 

 mind, and always use the glass circle if you wish to insure your 

 preparation against disappearance in a black sea of death with the 

 first hot spell. 



The subsequent proceedings are almost the same as those first 

 described in the present paper. The cell is to be made with the 

 large punch, so as to leave ample space for the rays of light from 

 the mirror to pass between its inside edges and the central stop, 

 upon which the specimen is to be mounted. If one thickness of 

 the paper or blotting pad be not sufficient, a second or third may 

 be pasted upon it, until the desired depth is reached. And, of 

 course, the covering paper for the back must be punched to allow 

 the light to pass up, and not be put on solid, as mounts for ordi- 

 nary opaque illuminations are. The object is best attached to the 

 glass circle by means of the gelatine cement, and the slide is to 

 be finished precisely the same as heretofore directed. And 

 finally, all fears of moisture spoiling a beautiful preparation in the 

 future may be dismissed as groundless. 



Our work thus far has been confined to opaque objects requir- 

 ing surface illumination, and it may be said that the great majority 

 of all to be mounted in the dry way are of this class. But many, 

 notably scales and hairs of insects, and plants, many diatoms, sec- 

 tions of pith, etc., are best viewed in the dry state and by 

 transmitted light. Most of these may be mounted upon the 

 cover, (the method of doing so in the case of diatoms, 

 having been given in a former paper), and thus may 

 be mounted in an ordinary cement cell without fear of 

 moisture. The most satisfactory cement for this purpose (and most 

 others), in my experience, is the white zinc, when properly prepared. 

 It dries quickly, has no tendency to run in, and makes a beautiful 

 finish to a mount. It should be used as follows, and the same direc- 

 tions will apply to asphalt or Brunswick black, if those cements be 

 preferred. 



Let us suppose that a ffe inch cover is to be used. Placing our 

 glass slip upon the turntable, we proceed to run a ring of cement 

 about its centre, the outer diameter of which shall be slightly in 



