The Microscope, 335 



medium, the thin and the extra thin. The microscopist may- 

 select either that he wishes, altliough it is well to take the 

 medium or the extra thin. If he thinks that at some time in 

 the future he will want to examine his preparations with high- 

 power objectives and with a modern, wide-angled sub-stage 

 condenser, then he should take the thin or the thinnest medium. 

 If the thick slips are selected, the action of the sub-stage con- 

 denser may be interfered with, as it may not be able to come 

 close enough to the object to focus the light upon it. Tke extra 

 thin slips are, to the writer, not so pleasant to handle as are 

 the medium. The latter are satisfactory and can be used with 

 a wide-angled condenser. The reader will do well to select 

 these, and to confine himself to one thickness, for the sake of 

 uniformity, if for no other reason. 



When the slips are received from the dealer they will not be 

 clean enough to be used in the delicate work of microscopical 

 mounting. They must be cleaned ; and here is a point that the 

 working microscopist should always remember. It is one of 

 the rules that have no exception. It is that the microscopist, 

 in all his manipulations, should be scrupulously neat. All his 

 slips and thin covers, as well as the objects, should be as clean 

 as clean can be. This is one of the charms as well as one of the 

 necessities of microscopical work. When the object is finally 

 mounted and is placed under the microscope, every little imper- 

 fection will become conspicuous, and every little speck of dirt, 

 every thread of lint, every fibre and bit of colored wool, every 

 little shred of wood or of cork cell will become more promi- 

 nent than the object itself, and may indeed obscure it. To 

 avoid all traces of these extraneous matters is often impossible, 

 but the careful microscopist is ever mindful of them and ever 

 on the alert to find and to dispose of them. Neatness is one of 

 the microscopist's cardinal virtues. His slips should then be 

 scrupulously clean. He can scarcely polish them too often, A 

 good way by which to ascertain if the glass surface is as clean 

 as it should be, is to breathe on it. The vapor from the breath 

 will condense on the surface, and if the glass is clean and 

 smooth the moisture will pass off without leaving momentary 

 and irregular traces ; it will melt away in an advancing and 

 regular wave of evaporation ; but if the surface is dusty or 

 imperfect, every grain of dust will, for a moment, be surrounded 



