232 SELECTION AND USE 



a few weeks, become very hard, and may be finished so as to 

 be very neat. When covered with a few coats of shellac var- 

 nish or pure gold size, and completely dried, 'they hold liquids 

 very well. They adhere so firmly to the slide that on several 

 occasions when the slide has been broken by a fall, the cell has 

 not parted from the glass. The only objection which we find 

 to them is the length of time which it takes them to harden. 



Tin foil, which may be had of various thicknesses from that 

 of thin paper to a sixteenth of an inch, makes a capital mate- 

 rial for cells. It is difficult to get the inner and outer circles 

 which form the ring, concentric, except by the use of special 

 tools. Prof. Chester avoids this difficulty by placing a large 

 number of rings on a rod or mandril which just fits the open- 

 ing, and after screwing the rings tightly endwise, he turns off 

 the outside so as to leave it perfectly true and even. Mr. A. Y. 

 Moore cements the sheet of tin foil to the slide by means of 

 shellac, and cuts out the ring on the turn-table by means of a 

 sharp knife or chisel. 



Finishing the Slides. The appearance of a collection 

 of slides depends very much upon the style in which they are 

 finished, and although in some instances it may be said that 

 the finish does not affect the intrinsic value of the object, it 

 is generally the case that a well-finished slide is more durable 

 than one that has not been properly completed. The old sys- 

 tem of covering the slides with paper is now entirely obsolete, 

 and properly so. It was troublesome, unsightly, unless in pro- 

 fessional hands, and not very durable. Fortunately slides 

 with ground and polished edges are now so cheap that there is 

 no occasion to resort to the paper cover. Objects mounted on 

 these slides, whether in cells or otherwise, are in general cov- 

 ered with round covers, which are adjusted on the turn-table 

 so as to be perfectly central. After the mount has been com- 

 pleted so far as fastening down the cover is concerned, the 

 edge of the latter is finished with a neat coat of varnish. This 

 varnish serves to do something more than merely ornament the 

 slide; it secures the cover in its place, and prevents the drying 

 up of the medium used for mounting. Even in the case of 

 Canada balsam it is of use, for if gold size be used as the var- 



