COVERING GLASS 



439 



the slips vary considerably in thickness, it will be advantageous to 

 determine on a gauge for thin, thick, and medium glass. The first 

 may be employed for mounting delicate objects to be viewed by the 

 high powers with which the apochromatic and achromatic condensers 

 are to be used, so as to allow plenty of room for the focal point of an 

 optical combination with great aperture to be fixed readily upon the 

 plane of the object; the second should be set aside for the attach- 

 ment of objects which are to be ground down, and for which, there- 

 fore, 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 solution. 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 thoroughly rinsed. 

 (The same solution may be advantageously used for cleansing cover- 

 glasses.) 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 washed in methylated spirit to ensure freedom from 

 greasiness. 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 sodaB, 

 finishing with clean water. 



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 is entirely superseded by the thin glass manufactured by Messrs. 

 Chance, of Birmingham, which may be obtained of various degrees of 

 thickness, down to the --^th of an inch. This glass, being unannealed, 

 is very hard and brittle, and much care and some dexterity are re- 

 quired in cutting it ; hence covers should be purchased, as required, 

 from the dealers, who usually keep them in several sizes and supply 

 any others to order. Save the fact that ' cover-glass ' is made by 

 Messrs. Chance, there is no definite information as to the mode of its 

 manufacture and the conditions upon which it is most satisfactorily 

 .produced. It w r ould be an advantage to the microscopist to possess 

 information on this point. The different thicknesses are usually 

 ranked as 1.2, and 3 ; the first, which should not exceed in thickness 

 the '006 in., being used for covering objects to be viewed with low 

 powers ; the second, which should not exceed '005 in. in thickness, 

 for objects to be viewed with medium powers ; and the third, which 

 ought never to exceed '004 in. in thickness, for objects which either 

 require or may be capable of being used with high powers. It must, 

 however, be remembered that the achromatic objectives of great 

 power and great aperture (1*5) will require much thinner covers 

 than even this. The thinnest glass is of course most difficult to 

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



