26 PREPARATION AND MOUNTING 



may be employed, which thoroughly and speedily cleanses 

 glass from all traces of grease or balsam. 



We have before said that any object to be viewed in the 

 microscope must have its support; but if this object is to 

 be preserved, care must be taken that it is defended from 

 dust and other impurities. For this purpose it is necessary 

 to use some transparent cover, the most usual at one time 

 being a plate of mica, on account of its thinness ; this sub- 

 stance is now, however, never used, thin glass being substi- 

 tuted, which answers admirably. Sometimes it is required 

 to '' take up " as little space as possible, owing to the short- 

 ness 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, and 

 probable defect of due annealing, it is difficult to cut, as it 

 is very liable to "//y" 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 conse- 

 quently to bear the pressure required in cutting the covers. 

 The process of cutting being so difficult, especially with the 

 thinner 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 contrivances 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 

 npt to become rough at the edge when much used. A friend 

 of mine uses thin brass plates with circles of various sizes 



