The Microscope. 273 



The cii'cular disks as well as the squares vary in size. The 

 former may be had from three-sixteenths to seven -eighths inch 

 or more in diameter, and the squares frotu one-half to one inch. 

 The smallest circles are useful for mounting a single minute ob- 

 ject, such, for instance, as a single Diatom, but for ordinary use 

 flve-eighths inch in diameter is a good size. Few permanent 

 mounts are made with squares, but for temporary purposes, 

 where the object is studied but not kept, or where it is living 

 and moving, a large square is the most useful and convenient 

 form for the cover glass. Its corners project beyond the margins 

 of the cement or other kind of cell enclosing the object, so that 

 as the water evaporates it may be easily supplied by adding a 

 drop at one corner, when it will gently flow under by capillary 

 attraction. There is no objection to the permanent mounting 

 of a slide with a square cover glass, except that a circle looks 

 better, and since the cell is usually circular, a circular cover fits 

 it better and the finishing cement may be more neatly and expe- 

 ditiously applied at the margin. For sections of some large 

 object, or for a number of sections to be mounted in regular 

 sequence as cut by the microtome, squares or oblong pieces are 

 much more desirable than the circles, and are pretty generally 

 used for this purpose by microscopists. The optician will cut 

 them to order, if desired. 



The thin sheets of glass, so brittle that they break almost at 

 a word or a look, are cut with a diamond. The circles are made 

 by placing on the glass plate pierced with holes somewhat larger 

 than the disks desired, and a diamond is then run around inside 

 these openings until the entire surface of the glass sheet is cov- 

 ered with the scratched circles. To attempt to break these out 

 would be followed by disastrous consequences, but if the glass 

 be laid aside for a day or two, the disks will fall out of their 

 own accord. The squares are cut with a diamond and a ruler, 

 after the thin sheet has been attached by water to a flat surface, 

 usually of plate glass. After the cutting the squares are easily 

 broken off by sliding the sheet to the edge of the support. 



As the covers come from the dealers, they are usually in any 

 but a clean condition. The surface is commonly obscured by 

 the results of the handling to which the glass is necessarily 

 subjected, and this greasy portion must be entirely removed 

 before the cover can be used. Some microscopists soak their 

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