42 
ELEMENTARY CHEMICAL MICROSCOPY 
may be whose presence will be revealed by their diffraction 
patterns. When the upper limit of obliquity is reached the 
illuminators are usually designated as iiltracondensers and the 
instruments to which they are attached are then known as 
uUramicroscopes. There is no sharp dividing line between 
ordinary dark-ground illumination and ultramicroscopic illu¬ 
mination; the one gradually merges into the other. In all ultra¬ 
microscopes we are dealing with dark-ground illumination, but, 
on the other hand, few dark-ground illuminators yield light rays 
sufficiently oblique to demonstrate particles of ultramicroscopic 
size. Typical ultracondensers are shown in Fig. 19. A com¬ 
parison of the indicated light ray directions in these with those 
Fig. 19. Types of Reflecting Condensers for the Study of Ultramicroscopic 
Particles. 
in Fig. 14 will disclose that their inclination is considerably 
greater. For the chemist the ultracondensers are of far more 
value than simple dark-field illuminators.^ 
The Adjustment of Dark-field Illuminators for use requires 
close attention, chiefly, to five conditions: (i) a selection of a 
sufficiently powerful radiant and the projection of a spot of 
light large enough to completely fill the lower opening of the 
illuminator; (2) the employment of objectives having a numer¬ 
ical aperture never greater than 0.90; (3) the use of object slides 
of the thickness for which the illuminator has been designed; 
(4) accurate centering of the illuminator with respect to the 
1 Dark-field illuminators are manufactured by the Spencer Lens Co., Buffalo, 
N. Y., and the Bausch & Lomb Optical Co. of Rochester, N. Y. That made by 
the latter firm is preferable for the chemist, yielding more brilliant preparations 
and disclosing the presence of finer colloidal suspensions. 
