ILLUMINATION OF OBJECTS; ILLUMINATING DEVICES 45 



and will require extreme care in adjusting the illumination and 

 in selecting the proper objectives. 1 



It is evident that with a properly selected optical combination, 

 the field of view will appear black or very dark, while any objects 

 present will appear to be bright and self-luminous. 



The more oblique the rays the more minute the particles 

 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 

 ultramicroscopes. 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 



FIG. 20. Types of Reflecting Condensers for the Study of Ultramicroscopic 



Particles. 



size. Typical ultracondensers are shown in Fig. 20. A com- 

 parison of the indicated light ray directions in these with those 

 in Fig. 15 will disclose that their inclination is considerably 

 greater. For the chemist the ultracondensers are of far more 

 value than simple dark-ground illuminators and those fitting into 

 the substage will be found preferable to those of plate form, 



1 Siedentopf and Zsigmondy have shown (Ann. d. Phys. [4] 10 (1903), 14) that 

 in the ultramicroscope the brilliancy of the diffraction disks is proportional to the 

 product of the squares of the numerical apertures of the image-forming and illu- 

 minating objectives. 



