THE MICROSCOPE. 



wards from the surface oi the plane mirror, which being received 

 upon the concave face of the Heberkuhn, are in turn reflected down- 

 wards upon the object under view. The foci of the objective and 

 Heberkuhn being coincident, it follows that when the specimen 

 under view is brought precisely into that of the former, its illumina- 

 tion by the latter is at the best. The central rays of light, how- 

 ever, coming immediately beneath the object, must be 

 stopped out by some opaque background to insure the best effect. 

 There are many modes of effecting this, and most of them 

 involve the use of the various cells, which are hermetically sealed. 

 There having been described at length by many more capable writers, 

 I shall confine myself to the one method whereby our porous paper 

 medium may be employed. For this purpose we shall need no addi- 

 tional tools or materials, save a large punch, say _% inch, and some 

 small circles of thin glass of ^ to Vg inch in diameter. Placing a 

 glass slip upon the two tubes, we proceed to paint a disc, (very 

 slightly larger than the circle of glass to be used), exactly in its cen- 

 tre. This is to be done with asphalte or Brunswick black, and the 

 slide set aside for the same to harden, which it will do in an hour or 

 two, or, if necessary, may be hastened by heating gently over the lamp 

 or on the brass table. A second coating of the asphalte is now to 

 be applied, and a circle of thin glass slightly warm is to be placed 

 upon its surface with the forceps and gently pressed down to ex- 

 clude air and cause perfect adhesion over its entire under surface. 

 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 disappearences 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 paper 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 



