ULTRAMICROSCOPY 61 



the greater concentration of the light but on account of its 

 greater resolving power enabling us to separate objects which 

 with the naked eye would be irresolvable. 



Though there are very few ultramicroscopic particles which 

 are self-luminous, 1 yet they can often be made virtually self- 

 luminous by illuminating them with an intense beam of light. 

 When such a beam is incident upon a small opaque object, 

 diffraction occurs ; the shadow cast by such a body is not 

 what would be expected from the theory of geometrical 



Fig. I. — Diffraction of light by an ultramicroscopic particle ; orthogonal arrangement of 

 the axes of the illuminating (drawn full) and diffracted (dotted) rays. 



optics ; light is given out in all directions, the intensity of 

 the light depending on the direction. The small object will 

 act like a self-luminous particle, and it may become visible 

 by reason of this diffracted light (see fig. i). 



There are many familiar examples of this action. When a 

 beam of sunlight enters a dark room, the path of the beam 

 is shown by the light diffracted by the small dust particles 



1 A striking example of very small self-luminous objects rendered visible under 

 such circumstances is afforded in the spinthariscope of Crookes. The small par- 

 ticles shot out from the radium salt strike a screen of zinc sulphide and give rise 

 to phosphorescence. The small patches of light stand out on a dark background. 



