186 PHOTOMICROGRAPHY 



contrast produced by dark ground illumination, between 

 the brightly shining object and the black field, is so great, 

 that minute bodies, not otherwise visible, can be seen very 

 distinctly. This has given rise to the application of the 

 term " Ultramicroscope " to the dark ground condensers, 

 but this is really a misnomer, as the objects for which 

 they are intended are well within the ordinary range of 

 visibility if properly displayed. Ultramicroscopy properly 

 refers to much more minute particles, such as are made 

 visible in colloidal solutions by a special application of 

 the same principle of contrast between bright object and 

 dark background, brought about by illumination by a 

 beam of light at right angles to the optical axis of the 

 microscope, in the method devised by Siedentopf and 

 Szigmondy. 



Radiants for High-Poiver Dark Ground Photomicro- 

 graphy. The contrast necessary to display 7 blood cor- 

 puscles, bacteria, and other minute bodies, for which the 

 new condensers are particularly useful, sufficiently 

 brilliantly for photography, is only produced by very 

 intense illuminants, more especially as the particles are 

 generally in fairly rapid movement in a fluid medium. 

 Exposures of about ^V or -^V of a second are then the 

 longest permissible, and are only possible with sunlight, 

 arc -light, or limelight ; exposures of 1 to 5 seconds are 

 required by Nernst lamp or incandescent gas combined 

 with a good condensing system. A cooling trough is 

 also frequently essential on account of the deleterious 

 effect of heat on the living organisms. 



Adjustments for the Dark Ground Illuminators. A 

 vertical camera is generally the most convenient on 

 account of the movements produced in a liquid prepara- 

 tion when placed on a microscope in the horizontal 

 position. The mirror is centred w r ith great care, the 

 condenser is oiled, placed in position, and accurately 

 adjusted. This is effected by racking it up until the 



