202 MOUNTING OF OBJECTS. THIN GLASS. 



instance to pieces of very thick plate-glass ; only transferring them 

 to the ordinary slides when they have been reduced to nearly the 

 requisite thinness (§ 161). 



149. Thin Glass. — The older Microscopists were obliged to em- 

 ploy thin laminae of talc for covering objects to be viewed with 

 lenses of short foci ; but this material, which was in many respects 

 objectionable, is now entirely superseded by the thin-glass manu- 

 factured for this express purpose by Messrs. Chance of Birmingham, 

 which maybe obtained of various degrees of thickness, from l-20th 

 to l-250th of an inch. This glass, being unannealed, isveryhard and 

 brittle ; and much care and some dexterity are required in cutting 

 it. This should be done with the writing diamond ; and it is ad- 

 vantageous to lay the thin glass upon a piece of wetted plate-glass, 

 as its tendency to crack and ' star ' is thereby diminished. For 

 cutting square or other rectangular covers, nothing but a flat rule 

 is required. For cutting rounds or ovals, on the other hand, it is 

 necessary to have 'guides' of some kind. The simplest, which 

 are as effective as any, consist of pieces of flat brass-plate, per- 

 forated with holes of the various sizes desired, or curtain-rings, 

 with a piece of wire soldered on either side : these being held 

 firmly down on the thin glass with two fingers of the left hand, 

 the writing-diamond is carried round the inner margin of the aper- 

 ture with the right ; care being taken that, in so doing, the diamond 

 be made to revolve on its own axis, which is needful both that it 

 may mark the glass, and also that the beginning and the end of the 

 cut may join.* Where a number of such ' rounds ' are being cut 

 at once, it saves much trouble, as well as risk of loss by breakage, 

 in separating them, to cut the glass first into strips whose breadth 

 shall equal the diameter of the rounds. But it is very convenient 

 to use-up for this purpose any odd pieces of glass whose shape may 

 render them unsuitable for being cut into ' squares' without much 

 waste. The pieces of thin glass thus prepared for use should be 

 sorted, not only according to size and shape, but also according to 

 thickness. The thinnest glass is of course most difficult to handle 

 safely, and is most liable to fracture from accidents of various 

 kinds ; and hence it should only be employed for the purpose for 

 which it is absolutely needed, namely, the mounting of objects 



* A very elegant little instrument, for the purpose of cutting thin- 

 glass rounds, contrived by Mr. Shadbolt, and another, of a more substan- 

 tial character, invented by Mr. Darker, will be found described hi Mr. 

 Quekett's " Practical Treatise." These instruments, however, are rather 

 adapted for the use of those who have occasion to prepare such rounds 

 in large quantities, than for the ordinary working Microscopist, who will 

 find the method above described answer his requirements sufficiently 

 well. Indeed it is in some respects superior ; since a firm pressure made 

 by the ring or plate on the glass around, tends to prevent the crack from 

 spreading into it. But to every one to whom the saving of time is a greater 

 object than the expenditure of a few shillings, it is strongly recommended 

 that these ' rounds ' should be purchased ready cut ; as they may be 

 obtained of any required size and thinness, at a very moderate cost. 



