48 ELEMENTARY CHEMICAL MICROSCOPY 



until the spot of light attains its smallest size. Focus upon this 

 tiny spot with a low power objective; if the condenser is properly 

 centered the spot will lie at the center of the field. Should it 

 lie to one side, bring it to the center by means of the centering 

 screws or center the objective with respect to the point of light. 



Having adjusted the condenser, the next step, if the device 

 is of the cardioid type (see page 67), is to ascertain whether 

 the quartz cell, which must be used with the instrument, is in 

 proper condition for use. Lay the quartz cover upon the cell 

 and press it down very carefully. Notice whether there appears 

 at the zone of contact between cell and cover a series of colored 

 concentric rings. If the pattern does not consist of concentric 

 circles, but appears to be elliptical, it is probable that the cell is 

 not level with respect to the optic axis. Adjust the level screws 

 until the plane of the cell is normal to the optic axis. If the 

 eccentricity of the rings does not disappear, the trouble lies in 

 the objective which is not corrected for the thickness of the cover 

 of the cell being used. 



A powerful source of light is essential. Direct sunlight by 

 means of a clockwork heliostat is ideal but seldom available. 

 The next choice is an electric arc of 4 to 5 amperes or more, for 

 ordinary dark-ground examinations, and of 15 to 20 amperes for 

 ultramicroscopic studies of colloids, etc. Useful types of radi- 

 ants will be found described on page 132. 



The more powerful the radiant the smaller the particles 

 which can be demonstrated. Siedentopf estimates that direct 

 sunlight will reveal the presence of particles whose diameters are 

 one-thirtieth of that of the smallest appreciable with the ordinary 

 arc lamp. 



Since the light rays enter these reflecting condensers through 

 an annular space, there being an opaque stop at the center, it is 

 obvious that the spot of light reflected from the mirror of the 

 microscope must have a diameter slightly greater than this 

 space, otherwise the illuminator will not properly function; for 

 this reason, before placing the illuminator in position for cen- 

 tering, it is always essential to examine its lower surface and 

 ascertain the diameter of the spot of light necessary to completely 



