APPARATUS 17 



nice they may look. Leave them overnight or 24 hours in cleaning 

 fluid: 



Bichromate of potash 20 g. 



Sulphuric acid 30 c.c. 



Water 250 c.c. 



Rinse well in clean water and then leave them overnight or 24 hours 

 in soapy water with plenty of soap. Rinse thoroughly in clean water 

 and wipe dry. The cleaning fluid cleans. The soap is necessary on 

 account of the acid in the cleaning fluid, and rinsing removes the 

 soap. Unless the acid is completely removed, many stains will fade. 



There is never any objection to very thin covers, except that they 

 require care in cleaning. For mounts which are to be used with an im- 

 mersion lens, it is better to have the cover of the same width as the 

 slide. The advantage is evident, since there is no danger of getting 

 balsam on the cover when wiping off the immersion fluid ; besides, one 

 can put sections to the very edge of the slide and still be sure that they 

 will be covered. Since most mounts for research work are mounted 

 under long covers and are intended for examination with immersion 

 lenses, we should recommend covers of 25X50 mm., or even 25X60 

 mm. Round covers are desirable only when mounts are to be sealed 

 on a turntable. Larger slides and correspondingly larger covers are 

 needed for special purposes. 



By consulting a catalogue, which will be furnished by any dealer, 

 the beginner can determine what he needs to buy, and what he can 

 find substitutes for, if it is necessary to be very economical. 



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