OP THE MICROSCOPE. 159 



As procured from the manufacturers, the slides are always 

 dirty, never having been washed after the process of grinding 

 and polishing the edges. If this dirt were soft it would not 

 matter so much, but it is in general hard and gritty being in 

 fact the grinding sand and the consequence is that the surfaces 

 of the slides are very apt to be scratched and injured. There 

 is but one firm that exports slides to this country, and they are 

 very careless in this respect. Out of a gross of slides it is often 

 difficult to find two dozen that are not so scratched as to be 

 worthless for the finest class of work. Having procured the 

 slides, however, the first thing to do is to clean and assort 

 them. They should be cleaned by being rinsed in water con- 

 taining a little washing soda; the dirt being removed if neces- 

 sary by the use of an old nail brush or tooth brush. Until 

 this has been done they should not be wiped with cloth or 

 leather, for by so doing the particles of sand are dragged along 

 the surface, making a deep mark. They should then be washed 

 in pure water, carefully wiped with a soft cloth, and assorted 

 for thickness and quality. It is in general best to sort them 

 into three classes thick, medium and thin the latter being 

 used for test and other very delicate objects. Elaborate instru- 

 ments have been devised for measuring the thickness of the 

 slides, so as to assort them accurately, but they are entirely un- 

 necessary; the eye is a sufficiently accurate guide. To deter- 

 mine their quality, they must be examined under the micro- 

 scope, and as it is only the central portion that is of any con- 

 sequence in this case, we place them on a brass plate, 3 by 1, 

 with the edges slightly turned up, and having a hole five- 

 eighths of an inch in diameter in the centre. That part which 

 lies over the hole is the only part which it is necessary to ex- 

 amine. Slides which contain air-bubbles, striae or scratches, 

 are at once laid aside to be used either for opaque objects or 

 those of a very coarse kind. Those that are perfect are care- 

 fully stored away where they will not be subject to injury. 



COVERS. After being properly arranged on the slide with a 

 suitable preservative medium, the objects must be covered with 

 a small piece of thin glass. Glass intended specially for this 

 purpose is made in England, and imported either in sheets or 



