210 SELECTION AND USB 



cut into squai-eg and circles of suitable sizes. Directions foi 4 

 cutting these covers would be out of place here. The beginner 

 will always iindit most economical to buy them ready cut. Of 

 the two kinds round and square the former are, for all ordi- 

 nary purposes, the most convenient, as covers of this shape are 

 best suited to cells made with the turn-table, and they may also 

 be finished more easily and neatly than the square ones. 



Covers should be carefully assorted for thickness, since the 

 thickness of the cover exerts a material influence on the per- 

 formance of all lenses except those of the lowest power or 

 quality. Where objectives which do not adjust for thickness of 

 cover are employed, the microscopist should find out the exact 

 thickness to which they have been corrected by the maker, and 

 use glass of this thickness in covering all objects that are to be 

 examined by means of these lenses. 



The inexperienced student will be apt to find some difficulty 

 in cleaning these covers. They are so fragile that it is difficult 

 to rub them, so as to remove dirt, without breaking them. 

 The best method is to soak them in a weak solution of potash, 

 rinse them off carefully several times with clean water, and after 

 pouring the last water off, give them a final rinsing by taking 

 them up in a pair of forceps and moving them about in a tum- 

 bler of clean water. They should then be laid (singly, of 

 course) on a wiping block and wiped. Wiping blocks are made 

 by covering a flat block of wood with chamois leather or linen, 

 drawn tightly so as to present a flat but somewhat soft surface. 

 These blocks are generally made round and with handles, but we 

 prefer them oblong (4 by l inches) and without handles. One 

 of them is laid on the table face up ; upon this face the thin 

 glass is laid and wiped with the other block. In this way the 

 thinnest glass can be cleaned without risk of fracture. 



CELLS TURN-TABLE. All objects that are mounted dry or in 

 fluid should be placed in cells, as unless this is done it is dif- 

 ficult to arrange the object properly or to secure the thin cover 

 permanently. In the majority of cases these cells consist of 

 little more than a ring of cement laid on the glass slide and 

 allowed to harden, and their depth does not exceed the thick- 

 ness of a sheet of paper. Such cells are in constant demand, 



