OF MICROSCOPIC OBJECTS. 11 



owing to the shortness of focus of the object-glasses. It can be 

 procured of any thickness, from one-fiftieth to one-two-hundred- 

 and-fiftieth of an inch. On account of its want of strength it is 

 difficult to cut, as it is very liable to "fly " from the point of the 

 diamond. To overcome this tendency as much as possible, it 

 must be laid upon a thicker piece, previously made wet with 

 water, which causes the thin glass to adhere more firmly, and con- 

 sequently to bear the pressure required in cutting the covers. 

 The process of cutting being so difficult, especially with the thin- 

 ner kinds, little or nothing is gained by cutting those which can 

 be got from the dealers, as the loss and breakage is necessarily 

 greater in the hands of an amateur. It is convenient, however, 

 to have on hand a few larger pieces, from which unusual sizes 

 may be cut when required. 



If the pieces required are rectangular, no other apparatus will 

 be required save a diamond and a flat rule ; but if circles are 

 wanted, a machine for that purpose should be used (of which no 

 description is necessary here). There are, however, other con- 

 trivances which answer tolerably well. One method is, to cut out 

 from a thick piece of cardboard a circle rather larger than the 

 size wanted. Dr. Carpenter recommends metal rings with a 

 piece of wire soldered on either side ; and this, perhaps, is the 

 best, as cardboard is apt to become rough at the edge when 

 much used. A friend of mine uses thin brass plates with circles 

 of various sizes " turned " through them, and a small raised han- 

 dle placed at one end. The diamond must be passed round the 

 inner edge, and so managed as to meet again in the same line, 

 in order that the circle may be true, after which they may be 

 readily disengaged. The sizes usually kept in stock by the 

 dealers are one-half, five-eighths, and three-quarters inch diameter ; 

 but other sizes may be had to order. 



For the information of the beginner it may be mentioned here 

 that the price of the circles is a little more than of the squares ; 

 but this is modified in some degree by the circle being rather 

 lighter. If appearance, however, is cared for at all, the circles 

 look much neater upon the slides when not covered with the 

 ornamental papers ; but if these last are used (as will shortly be 

 described) the squares are equally serviceable. 



