176 THE USE OF THE MICROSCOPE 



cleansing them, alcohol, xylol, or water should be kept away 

 from the edges, which may well be coated with a shellac 

 varnish. These mounted gelatin films should not be 

 left about on the table, perhaps in direct sunlight, but 

 should be put away in a box when not in use. Similarly, 

 the turning out of the lamp, when not in use, will extend 

 the duration of the gelatin color filters. These are pref- 

 erable to the colored glasses, because of the large number 

 available with different absorptions; because they can be 

 accurately reproduced; and because they are obtainable, if 

 necessary, in plane-parallel glass (though optically worked 

 Chance colored glass is now procurable). They should 

 be handled by the edges or corners. 



Care of the Reflecting Prism, or Plane Mirror. — The 

 surfaces of these are optically plane, and they should be 

 treated with as much care as the surfaces of condenser 

 lenses. Hydrofluoric acid, or pastes containing it, are 

 sometimes regularly kept and used in the biological labora- 

 tory to dull glass surfaces for writing on. They should 

 not be in the same room with the microscopes, for the 

 vapors may spoil all exposed surfaces of optical glass, 

 especially if a containing bottle (often made of paraffin wax) 

 gets uncorked or broken. Dust in some localities contains 

 particles of quartz which will scratch glass. Hence, it 

 should be blow^n off the plane mirror or prism, before wiping 

 this with lens paper. 



Care of the Condenser. — The condenser is usually 

 more exposed than the objective. In damp, hot cHmates, 

 or in warm, wet weather, fungus filaments may grow over 

 the glass, and will mark or dim the surface of the flint 

 glass if not regularly wiped off. The top lens of a condenser 

 may be held in by shellac cement, and alcohol must in 

 consequence be kept from it. In some makes of immersion 

 condenser (such as Watson's holoscopic), the top lens is 

 cemented to a thinnish plane-parallel glass disc, w^hich may 

 be cracked by rough usage. Condensers with large lenses 

 should not have the electric bulb too close, or they may 

 heat up or crack. A paper scale may well be fixed close 



