30 SOME HINTS ON THE PREPARATION AND 



Illumination by means of a lieberkuhn, which throws the light 

 directly down upon the object without shadows, has been too much 

 neglected. In England it is a very ordinary method of viewing 

 opaque objects, and a lieberkuhn is usually furnished with every ob- 

 ject-glass from a three inch to a four-tenths. But it is different 

 here, and I am quite sure that the great majority of our observers, 

 professional and amateur, are totally unacquainted with its use. I 

 think this is to be regretted, since they lose much, both in pleasure 

 and instruction from its non-employment. Hoping that its use may 

 become more general, I will give a hint or two as to the method of 

 mounting a preparation, to be examined by this form of illumina- 

 tion. 



The lieberkuhn is a concave speculum fitting over the mounting 

 of the objective so that the front lens of the latter projects through 

 the middle of the lieberkuhn. Parallel rays of light are thrown up- 

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

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

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

 lieberkuhn 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 sea ? ed. 

 These 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 fa inch, and some 

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

 glass slip upon the turn table, 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. 



