116 
ELEMENTARY CHEMICAL MICROSCOPY 
justing by the substance to be studied. The only adjustment 
which should now be required will be the diameter of the slit; 
if there appears to be required a marked change in slit diameter 
it is probable that following this change there may be required 
slight changes of Vi and V2. 
If the beginner will proceed as indicated little difficulty will 
be experienced in adjusting the slit ultramicroscope for use. 
The most annoying feature is the change in the position of the 
crater of the electric arc, and consequently unequal illumina¬ 
tion of the sht results or there is a failure (due to a flickering arc) 
of the spot of light to remain centered upon the slit. Holding 
the black screen against the lens C2, on the side toward the slit, 
from time to time, will show when the arc needs adjusting, 
since there should appear a spot of light of uniform intensity 
and in the proper position to fall concentric with the optic axes 
of Cl, C2, C3. 
When dealing with exceedingly fine colloidal particles it is 
often an advantage to cut off the lower half of the beam by 
means of a screen mounted upon a saddle stand and placed 
between S and C2, the upper horizontal edge of the screen being 
raised so as to cut off the lower half of the beam of light. Ap¬ 
proximately as good results may be obtained more easily by laying 
against the end of the tube T a small rectangular piece of black 
hard rubber or blackened brass d, as shown in the diagram. 
Reflecting Condenser Ultramicroscopes consist of highly 
perfected dark-ground illuminators applied to ordinary micro¬ 
scopes provided with special objectives of low numerical aperture. 
In the special condensers used, the light rays are reflected from 
two spherical surfaces. The illuminating rays therefore enter 
the preparations with obliquities greater than in ordinary dark- 
ground illuminators and are brought to a correct focus.^ 
By employing objectives of low numerical aperture (about 
0.85) we have rays including only a low range of apertures taking 
1 In the ordinary paraboloid condensers, when properly constructed, the light 
rays are also brought to a focus, but the focal length varies from zone to zone, 
hence we have an overlapping of images at the center. (Zeiss, “Mikro” Circular 
306, p. 8.) 
