THE MICROSCOPE. 165 



a small scrap of paper pasted upon the slide will answer every prac- 

 tical purpose. The surface of the table should be sharply scratched 

 at the angle formed by the guide-plate, so that in the event of 

 having to move the latter to accommodate a slide not originally 

 ringed by it, the centering adjustment may be returned to its ori- 

 ginal position in a moment and with absolute accuracy. My original 

 device was cut from a piece of stiff card board, and secured to the 

 table by means of the screws which attached the two spring clips to 

 the latter; and this primitive affair did excellent service for many a 

 long year, ere it gave way to my present truly iT//"-centering instru- 

 ment. 



Let us take breath, for we have been a long while getting our 

 turn table; but that invaluable instrument secured, at last we are 

 ready for business. Taking a clean slide from our store box of same, 

 we slip it beneath the clips of the table, and bring it to a centre at 

 once, if we have followed the foregoing directions. First dusting 

 its surface carefully with a camel's hair brush, we proceed to trace a 

 flattened ring thereon, with balsam, diluted by chloroform to the 

 consistency of city cream, using a small red sable brush for the pur- 

 pose; ample directions for doing which were given in the preceeding 

 paper. The size of this ring must be the same as that of the cell we 

 are about to place upon it, and the concentric lines turned upon the 

 surface of the table, which are visible through the slide, fvirnish us 

 with an excellent guide to follow. ^^'hich of the three thicknesses 

 of sheet wax we shall use, of course, will be determined by the thick- 

 ness of the specimen to be mounted: if none, (used singly,) will 

 furnish a cell sufficiently deep we must add one or more rings of the 

 wax, taking care to coat each thoroughly with the balsam before 

 adding the next ring. This may be all done, without removing the 

 slide from the turn table, and thus a symmetrical and accurately 

 centered cell may be built up in a very few minutes; and if neces- 

 sary, may be at once filled with balsam and specimen, and the 

 mounting completed. It is better, however, to set it aside for a day 

 or two, (in a dust-tight case,) for hardening. .\nd now, having com- 

 pleted our cell, be it of glass, metal or wax, let us proceed to mount 

 our specimen therein, without a legacy of air bubbles, or vacuoles; 

 which is a very easy matter to do — when you once know how. 



Suppose that our first specimen is a lot of Foraminiferous shells 

 of considerable sii^e, which will recjuire a cell of the depth, made by 



