The Wonders of Microscopy 311 



in, so to speak, sideways, then particles ordinarily invisible might 

 stand out. Their diffraction-images, at any rate, would be seen. 

 In ordinary microscopic conditions the beam of light is thrown 

 by the mirror, usually through a sub-stage condenser, directly 

 through the solution or thin transparent section, up into the tube 

 of the microscope, where an image is formed, to be re-formed by 

 the eyepiece. In the ultra-microscope for examining solutions 

 the beam of light is projected horizontally into the solution and 

 examined from above. The result is that particles ordinarily in- 

 visible are seen in a vigorous dance, the so-called Brownian move- 

 ment. This dance is due to the particles being bombarded by the 

 moving molecules of the fluid in which they are suspended. By 

 accessory devices it becomes possible, in the use of the ultra-micro- 

 scope, to count the number of particles in a solution and to 

 measure the mass of each. This has formed the basis of exceed- 

 ingly interesting conclusions which are unfortunately beyond our 

 scope in this article. 



A reference should be added, however, to another method 

 called "dark-ground illumination" which makes structures visible 

 which are invisible in ordinary conditions of microscopic work. 

 Professor Bayliss writes: "The central rays of the illuminating 

 beam are cut out by means of a stop, and the peripheral rays are 

 reflected by a parabolic surface so as to meet in a point in the 

 object under examination; they cross at such an angle as to pass 

 outside the field of the objective in use, which only picks up the 

 light refracted, or diffracted, from structures in the preparation." 

 The dark -ground illumination brings out features which are in- 

 visible in the ordinary direct illumination. 



The essential parts of a microscope are, as we have seen, ( 1 ) 

 the objective for obtaining the first magnified image of the object; 

 (2) the ocular for further enlarging that image and transmitting 

 it to the observer's eye; and (3) the sub-stage condenser for il- 

 luminating the object with a cone of light. Now, in modern 

 times, there have been numerous detailed improvements in these 



