OF MICROSCOPIC OBJECTS. 25 
_ should, therefore, be taken to sort them, so that the more 
delicate objects with which the higher powers are to be 
used may be mounted upon the thinnest, as the light em- 
ployed in the illumination is then less interfered with. To 
aid the microscopist in this work, a metal circle may be pro- 
cured, having a number of different sized openings on the 
outer edge, by which glass slides can be measured. ‘These 
openings are numbered, and the slides may be separated 
according to these numbers; so that when mounting any 
object there will be no need of a long search for that glass 
which is best suited to it. 
When fresh from the dealer’s hands, these slides are 
generally covered with dust, &c., which may be removed by 
well washing in clean rain-water; but if the impurity is 
obstinate, a little washing soda may be added, care being 
taken, however, that every trace of this is removed by sub- 
sequent waters, otherwise, crystals will afterwards form upon 
the surface. Sometimes, however, a certain greasiness is 
very obstinate upon the glass. It is then necessary to use 
a little liquor potassze with a small piece of linen, rubbing 
the slide with some pressure, and then washing as before to 
remove all remains. A clean linen cloth should be used to 
dry the slides, after which they may be laid by for use. 
Immediately, however, before being used for the reception of 
objects by any of the following processes, all dust must be 
removed by rubbing the surface with clean wash-leather or 
a piece of cambric, and, if needful, breathing upon it, and 
then using the leather or cambric until perfectly dry. Any 
small particles left upon the surface may generally be 
removed by blowing gently upon it, taking care to allow no 
damp toremain. A very efficient remedy, also, is a mixture 
of equal parts of sulphuric ether and alcohol, with which 
the glass must be rubbed by the aid of a tuft of clean 
cotton-wool until no stain appears after breathing upon it. 
A strong infusion of nutgalls may be used in the same way, 
and is preferred by many to all other applications; or, a 
mixture of equal parts of alcohol, benzole, and liquor sodz- 
