204 CLEANING THIN GLASS. VARNISHES AND CEMENTS. 



when high powers are in use. It is undesirable to employ glass 

 covers of greater thickness than l-140th (*007) of an inch with 

 any object-glass whose aperture exceeds 75° ; and for object-glasses 

 of 120° and upwards the glass cover should not exceed 1 -250th 

 (•004) of an inch. 



151. On account of the extreme brittleness of the Thin Glass, 

 it is desirable to keep the pieces, when cut and sorted, in some fine 

 and soft powder, such as Starch. Before using it, however, the 

 Microscopist should be careful to clean it thoroughly ; not merely 

 for the sake of removing foulnesses which would interfere with the 

 view of the object, but also for the sake of getting rid of adherent 

 starch -grains, the presence of which might lead to wrong conclu- 

 sions, and also of freeing the surface from that slight greasiness 

 which, by preventing it from being readily wetted by water, fre- 

 quently occasions great inconvenience in the mounting of objects in 

 fluid. The thicker pieces may be washed and wiped without much 

 danger of fracture, if due care be employed ; but the thinner re- 

 quire much precaution ; and in cleansing these, the simple method 

 devised by Mr. Spencer will be found very useful. This consists in 

 the use of a pair of round flat disks, about 1| in. in diameter, 

 made of wood or metal covered with chamois leather, and fur- 

 nished with handles ; for when a piece even of the thinnest glass 

 is laid upon one of these, it may be rubbed clean with the other, 

 and any amount of pressure may be used without the least risk of 

 breaking it. Previously to doing this, however, it will be advan- 

 tageous to soak the pieces for a time in strong Sulphuric Acid, and 

 then to wash them in two or three waters ; but if greasiness be 

 their chief fault, they should be soaked in a strong infusion of 

 Nutgalls, with which it will be also advantageous to cleanse the 

 surface of glass slides that are to be used for mounting objects in 

 liquid. 



152. Varnishes and Cements. — There are three very distinct 

 purposes for which Cements that possess the power of holding firmly 

 to Glass, and of resisting not merely w r ater but other preservative 

 liquids, are required by the Microscopist ; these being (1) the 

 attachment of the glass covers to the slides or cells containing the 

 object, (2) the formation of thin cells of cement only, and (3) the 

 attachment of the glass-plate or tube-cells to the slides. The two 

 former of these purposes are answered by liquid cements or var- 

 nisJies, which may be applied without heat ; the last requires a 

 solid cement of greater tenacity, which can only be used in the 

 melted state. The varnishes used for mounting objects in liquid 

 should always be such as contain no mixture of solid 2>^'ticles. 

 This is a principle on which the Author, from an experience of 

 many years, is disposed to lay great stress ; having often made 

 trial, at the recommendation of friends, of varnishes which were 

 said to have been greatly improved by thickening with litharge or 

 lamp-black ; and having always found that, although they may 



