Microscopes 35 



the study of living organisms and other transparent materials which in- 

 herently have low contrast properties. By the use of special objectives 

 and other equipment a greater contrast between the transparent speci- 

 men and its surrounding medium is secured. 



Electron microscopy employs the use of electron (magnetic) micro- 

 scopes. Electrons produced by special apparatus are used instead of 

 light, magnetic fields ("electron lenses") are used instead of glass lenses, 

 and photographic plates are used to record the image since it cannot be 

 seen. The specimens being photographed must be very thin and in a 

 vacuum (Fig. 7). The fact that axially symmetrical magnetic and elec- 

 tric fields could be employed as lenses was discovered by H. Busch in 

 1926. Hence, by the proper use of magnetic fields (acting as lenses), 

 the charged particles (electrons) can be made to do what light waves 

 accomplish in ordinary, optical microscopy. Electron microscopes were 

 made by Knoll and Ruska (Germany) in 1932, by Marton (Belgium) 

 in 1934, and by Prebus and Hillier (Canada) in 1938. The Radio Cor- 

 poration of America in 1941 manufactured a commercial electron micro- 

 scope of the magnetic type. Many kinds and models have been made 

 • and used in various parts of the world since that time. Magnifications 

 of thousands of diameters are possible with such apparatus. 



QUESTIONS AND TOPICS 



1. Define the following types of microscopes: simple, compound, binocular, 

 monocular. 



2. List the important stages in the general history of the development of micro- 

 scopes, including the persons and their specific contributions. 



3. Explain how scientific progress may depend on the efficient use of microscopes 

 in such fields as medicine, agriculture, industries, chemistry, water purifica- 

 tion, metallurgy, and similar ones. 



4. Why can ultraviolet and electron microscopes not be used for viewing objects 

 with the eye ? 



5. What are chief differences between the following types of microscopes: light, 

 ultraviolet, electron ? 



6. How does the study of the history of the development of microscopes illus- 

 trate the use of the various stages of the so-called scientific method? Be as 

 specific as possible. 



7. How can a knowledge of the efficient use of a microscope be of value to us 

 in everyday life, even though we do not plan to follow a scientific career? 



8. Can one learn to use a microscope efficiently without some fundamental knowl- 

 edge of how the various parts of it really function? 



9. Is it true that a microscope can be no more efficient than the optical lenses 

 of which it is made ? 



