208 SELECTION AND TTSE 



utus. Small slides have this advantage, that they cost less, and 

 take up less room in a cabinet. Large slides look best, and af- 

 ford more room for descriptive labels, which is an important 

 point. But since slides 3 by 1 have been adopted by common 

 consent, the mtcroscopist who mounts specimens, or who buys 

 objects mounted on slides of a different size, commits a mis- 

 take for which the advantages offered by the small slides are 

 but a slight compensation. The only exceptions to this rule 

 are where the objects are too large to be mounted securely on a 

 slide of standard size, or where a large number are to be pre- 

 pared for the purpose of illustrating some special series of in- 

 vestigations. It is to be presumed that such a series will never 

 be broken up and separated, and as it will in all probability be 

 assigned to its own cabinet, it is sometimes of advantage to 

 have it upon slides of a size other than that in common use. 

 As the objects composing such a series will probably be num- 

 bered and catalogued, there is no necessity for extended de- 

 scriptions on the labels, and therefore slides of half the usual 

 size (l by 1) will serve very well. The cabinet may thus be 

 reduced in bulk by one-half. We have a special cabinet, illus- 

 trative of textile fibres, mounted upon slides of small size 3 and 

 find it quite convenient. 



The glass from which slides are cut should be free from air- 

 bubbles, scratches and that wavy appearance .which is due 

 either to inequalities in the surface or to irregularities in the 

 composition of the glass itself. Ordinary window glass is en- 

 tirely unfit for the purpose. The most suitable kind is plate 

 glass, the surface of which has been ground and polished, so as 

 to be perfectly even and smooth. Glass of this kind is used for 

 looking-glasses and by photographers, and when other material, 

 could not be had, we have made very excellent slides out of 

 broken looking-glasses and photographer's plates, though it is 

 difficult to get the latter thin enough. Slides of good glass 

 are, however, manufactured in quantity and sold at a reason- 

 able price, so that under ordinary circumstances it will hardly 

 pay the microscopist to cut out his own slides. Moreover the 

 slides sold by the dealers have the edges neatly ground, an 

 operation which the microscopist will find tedious and trouble- 

 some. 



