34 Introductory Biology 



special dark-field condenser (dark-field illuminator) whereby direct light 

 rays do not enter the specimen; the oblique light rays are focused on the 

 specimen^ which thus appears as a luminous body against a dark field. 

 A very small, bright object is more easily seen in a dark background 

 (field) than is a very small, dark object in a bright field. This is similar 

 to the phenomenon of seeing small dust particles in a beam of light 

 when the reo"ion back of the lioht beam is dark. 



1 



Fig. 7. — An electron microscope capable of magnifying thousands of times. (Cour- 

 tesy RCA Victor Corporation.) 



Phase microscopy was proposed by Zernike of Holland in 1932 and 

 has been studied by many, including Richards and others in America 

 (1944). Phase contrast is a new factor in image formation that permits 



