176 THE MICROSCOPE AND ITS REVELATIONS. 



by the high powers with which the Achromatic Condenser is to be used r 

 so as to avoid any unnecessary deflection of the illuminating pencil by 

 the thickness of the plate which it has to traverse beneath the object; the 

 second should be set aside for the attachment of objects which are to be 

 ground-down, and for which, therefore, a stronger mounting than usual 

 is desirable; and the third are to be used for mounting ordinary objects. 

 Great care should be taken in washing the slides, and in removing from 

 them every trace of greasiness by the use of a little soda or potass solu- 

 tion. If this should not suffice, they may be immersed in the solution, 

 recommended by Dr. Seiler, composed of 2 oz. of Bichromate of Potass, 

 3 fl. oz. of Sulphuric Acid, and 25 oz. of Water, and afterwards thor- 

 oughly rinsed. (The same solution may be advantageously used for 

 cleansing Cover-glasses, 132.) Before they are put away, the slides 

 should be wiped perfectly dry, first with an ordinary ' glass-cloth,' and 

 afterwards with an old cambric handkerchief. And before being used,, 

 each slide should be again carefully wiped, so*as to remove all adherent; 

 dust. Where slides that have been already employed for mounting 

 preparations are again brought into use, great care should be taken in 

 completely removing all trace of adherent varnish or cement; first by 

 scraping (care being taken not to scratch the glass), then by using an 

 appropriate solvent, and then by rubbing the slide with a mixture of 

 equal parts of alcohol, benzole, and liquor sodae, finishing with clean 

 water. 



166. Thin Glass. The older Microscopists were obliged to employ 

 thin laminae of talc for covering objects to be viewed with lenses of short 

 focus: but this material, which was in many respects objectionable, is 

 now only employed for Objectives of exceptionally short focus (such as 

 l-50th or l-75th inch), being entirely superseded for other purposes by the 

 thin glass manufactured by Messrs. Chance of Birmingham, which may be 

 obtained of various degrees of thickness, down to 1 -500th of an inch. This 

 glass, being un-annealed, 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 di- 

 minished. For cutting square or other rectangular covers, nothing but 

 a flat rule is required. The cutting of rounds bv unaccustomed hands is 

 usually attended with so much breakage, that it is really a saving of 

 money as well as of time to purchase them from the dealers; who usually 

 keep them in several sizes, and supply any others to order. The differ- 

 ent thicknesses are usually ranked as 1, 2, and 3; the first being used for 

 covering objects to be viewed with low powers, the second for objects to 

 be viewed with medium powers; and the third for objects requiring high 

 powers. 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. The thickest pieces, again, may be most advantageously em- 

 ployed as covers for large Cells, in which objects are mounted in fluid 

 ( 171-174) to be viewed by the low powers whose performance is not 

 sensibly affected by the aberration thus produced. The working Micro- 

 scopist will find it desirable to provide himself with some means of mea- 

 suring the thickness of his cover-glass; and this is especially needed if he 

 is in the habit of employing Objectives without adjustment, which are 

 corrected to a particular standard (17). A small screw-gauge of steel, 

 made for measuring the thickness of rolled plates of brass, and sold at 



