CARE OF THE MICROSCOPE 177 



to the iris handle of the condenser, with the apertures 

 marked on it. The appropriate stops for dark ground 

 should be fitted as close as possible to the lower focal plane 

 of the condenser. Cardboard flanges to hold them may be 

 easily cemented under the iris. These stops should be 

 kept handy in a small box. The top lens of a water- 

 immersed condenser must be kept free from traces of immer- 

 sion oil, or the water will not cling. Xylol may be used 

 instead of water with slides below the standard thickness. 



Care of the Slides. — The shdes should be nearly uniform 

 in thickness, and in color of glass. They can be cleaned 

 by acetic, chromic, or nitric acids. If well cleaned and not 

 touched with the fingers (which always exude some slight 

 grease), they can be used at once with a water-immersion 

 condenser. But otherwise, the under side of each must 

 be wiped before use with towelling paper or filter paper 

 moistened with xylol or alcohol, so that the water may 

 spread uniformly. Temporary preparations are often 

 to be preferred to permanent ones; and in that case the 

 slides may be used many times over. Here, especially, 

 there is a marked advantage in getting the slides uniform in 

 thickness, with a range of only 0.1 millimeter; for 1 milli- 

 meter is a convenient thickness, and such slides are not 

 often broken by the experienced worker, so that there is no 

 need for seeking thicker ones. If the corners are sharp, they 

 may be touched by a file wetted with xylol. 



Care of the Mounting Media. — Canada balsam in xylol 

 is conveniently kept in leaden collapsible tubes, when 

 used for mounting stained paraffin sections. Otherwise, 

 it is difficult to prevent it from fastening down the cork 

 stopper or cap. A glass cap, however, is easier to 

 keep clean than a stopper. Dammar is best made for 

 sealing by picking out the colorless lumps of the resin and 

 adding them in excess to a little xylol in a wide-mouthed 

 capped bottle. For closing iron-acetocarmine prepara- 

 tions, these are allowed to dry at the edges, and immediately 

 sealed with thick dammar applied with a brush (or with 

 melted paraffin). Where oil-immersion objectives are to 



