FIG. 10. Illuminated field as seen in ultramicroscope 



The rays converge to a focus in the centre and then diverge again, a and b. Note that the 

 greatest number of particles is rendered visible in the most brightly illuminated spot c. 

 This is due to the fact that the more intense the illumination the smaller are the particles that 

 i t is possible to observe. The particles which are too small to be seen outside the focus of 

 the beam are obvious under the more brilliant light at this focus (Zsigmondy). 



From " Principles of General Physiology," W. M. Bayliss (Longmans). 



