CUTTING THIN GLASS. 215 



of Birmingham, which may be obtained of various degrees of 

 thickness, from l-20th to l-250th of an inch. This glass, being 

 unannealed, is very hard and brittle ; and much care and some 

 dexterity are required in cutting it. This should be done with 

 the writing diamond ; and it is advantageous 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 rect- 

 angular 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, perforated 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 aperture 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. 1 Where a number of such u 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 

 lass is of course most difficult to handle safely, and is most 

 iable to fracture from accidents of various kinds ; and hence it 

 should only be employed for the purpose for which it is abso- 

 lutely needed, namely, the mounting of objects which are to be 

 viewed by the highest powers. The thickest pieces, again, may 

 be most advantageously employed as covers for large cells in 

 which objects are mounted in fluid ( 136, 137), to be viewed 

 by the low powers, whose performance is not sensibly affected by 

 the aberration thus produced. And the pieces of medium thin- 

 ness will be found those most serviceable for all ordinary pur- 

 poses ; neither being, on the one hand, difficult to handle ; nor, 

 on the other, interfering with the clearness of the image formed 

 by medium powers of moderate aperture, even when no special 

 adjustment is made for the aberration they produce ( 101). 



1 A very elegant little-instrument, for the purpose of cutting thin glass rounds, con- 

 trived by Mr. Shadbolt, and another, of a more substantial character, invented by Mr. 

 Darker, will be found described in Mr. Quekett's "Practical Treatise." These instru- 

 ments, however, are rather adapted for the use of those who have occasion to prepare 

 such rounds in large quantities, than for the ordinary working Microscbpist, 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 round, tends to prevent the crack from spreading into it. 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. 



