i66 THE MICROSCOPE. 



a ring cut from a single thickness of pond lily wax. Choosing such 

 an one from our stock of already prepared cells, — hold the slide very 

 carefully over the lamp, until it feels sensibly warm to the touch, 

 then immediately fill the cell with pure limpid balsam, just sufficiently 

 to reach the upper surface of same all around and to form a slight 

 convexity from side to side. The balsam may be squeezed from a 

 collapsible tube, or lifted from the supply bottle by means of a glass 

 rod, at pleasure; my own preference being decidedly for the former 

 method, as being cleaner, more convenient, and keeping the balsam 

 entirely free from dust, or evaporation. Now gently warm the slide 

 again to the .same degree as before and with a clean needle, — well 

 heated over the lamp, — explore the inner edges of the cell down to 

 its glass bottom, so that if there be any vagrant air bubbles impris- 

 oned thereabouts they may be freed, float to the surface and disap- 

 pear of their own volition, or at the touch of a hot needle point. A 

 careful examination of the cell's contents should be made with a 

 pocket lens, as a further surety that no imprisoned bubbles are left 

 therein. And it may be well to remark here and for all the past as 

 well as future operations, that they should be conducted speedily as 

 possible, since the balsam begins to harden as soon as brought into 

 contact with the air, rendering it more and more difficult to mani- 

 pulate, the longer the operation lasts. The bell glass should also 

 be close at hand to place over the slide during any pause in the 

 work, to exclude dust. 



The small shells, — previously freed from all moisture or dust 

 — may now be carefully dropped into the cell and moved into posi- 

 tion with a warm, nof hot needle, until the desired amount is deposit- 

 ed therein. Examine again to make sure no fugitive air bubbles 

 are left; and now we are ready for the covering in. The glass circle 

 for this purpose should be slightly smaller than the wax ring which 

 forms the cell: thus, if the diameter of the latter be % of an inch, 

 the former should be \\. The covers can be purchased in all sizes, 

 one-sixteenth of an inch apart, from ){ to i inch in diameter. 



Taking up the cleaned circle with the forceps and slightly 

 warming it over the lamp, we proceed to apply it to the cell precisely 

 in the same manner as previously described for a baLsam mount, in 

 which no cell was employed; care being taken to let it fall gently 

 so ris to drive out the excess of balsam over the c(\^i:i of the cell, and 

 so down on to the surfnce of the slide, wifluuit moving the shells out 



