ULTRAMICROSCOPY 



67 



the observing system the smaller ; dark ground illumination is 

 produced when the front lens of the illuminating system blocks 

 out the smaller aperture of the observing system from the cone 

 of illuminating rays. The specimen is illuminated with light 

 from apertures greater thano"2, and diffracted rays are employed 

 up to the same aperture. The functions of the two lens systems 

 can readily be seen from the figure. 



Fig. 5 shows Siedentopfs paraboloidal condenser. P is a 

 plano-convex piece of glass, the curved portion of which 

 represents a paraboloid of rotation. B is a central stop and 

 blocks out rays which have an aperture from o to ri (Zeiss). 

 The focus of the paraboloid is situated on the upper side of 



Fig. 5. — Path of rays in the paraboloidal condenser. 



the object-glass O. I is the immersion liquid between the slide 

 and the condenser. Dark ground illumination is produced by 

 total reflection at the upper surface of the coverslip when air 

 is above it. With this paraboloidal form better correction 

 for spherical aberration can be obtained, and a greater brightness 

 is produced. It is very convenient for observing, among other 

 objects, very small bacteria, and for taking instantaneous 

 microphotographs of the live bacteria. 



Having now described the principles involved and some of 

 the methods and apparatus employed in ultramicroscopy and 

 dark ground illumination, we will pass on to consider some of 

 the applications and the interesting results which have been 

 obtained by its use. 



