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 ultracondensers and the 

 instruments to which they are attached are then known as 

 ultramicro scopes. 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 



A B j| 



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. 1 



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. 



