350 COMPOUND AND ELECTRON MICROSCOPES 



plane just above the pole pieces of the projector lens. This first-stage 

 electron image is usually about 100 times as large as the object. 



Magnetic Projection Lens 



About 35 cm below the magnetic object lens lies the image projector 

 lens. This magnetic projection lens is used to produce the second-stage 

 magnification. The second-stage image lies in the final image-plane 

 40 cm below the pole pieces of the projector lens. 



If a small central portion of the first-stage electron image is further 

 magnified 100 diameters then a total magnification of 10,000 diameters 

 is attained in the second-stage electron image. This image may be 

 viewed through a side port in the instrument and seen in the image 

 plane as it excites a phosphorescent screen, or it may be reproduced by 

 allowing the electron beams to impinge on a photographic plate placed 

 in the plane of the second-stage electron image. 



Photographic Image 



The eye can distinguish detail in an object only when the detail in- 

 volves dimensions larger than a certain minimum value which at the 

 distance of distinct vision (25 cm) is about 0.074 mm. Its resolving 

 power is thus close to 14 lines per millimeter. The resolving power of a 

 photographic emulsion is defined as the number of lines of uniform width, 

 separated by spaces equal to their widths, which can just be distin- 

 guished per millimeter on the negative. The average emulsion has a 

 resolving power of about 40 or 50 lines per millimeter. A fine-grained 

 emulsion under most favorable development can attain a resolving 

 power of about 100 lines per millimeter. Magnification of a photo- 

 graphic image by a factor 10 therefore would not destroy the clarity of 

 an image on account of the granular structure of the emulsion. 



In the RCA electron microscope the magnification of the second-stage 

 image is about 10,000 to 16,000 diameters. The full useful magnification 

 may then be obtained by enlarging the photographic plate from six to 

 ten times, giving a total magnification of more than 100,000 diameters. 



The photographic response of the silver bromide gelatine emulsion 

 to the electron bombardment continues throughout the emulsion as long 

 as the penetrating electron has appreciable energy; thus the depth of 

 penetration of the electrons into the emulsion will determine the prac- 

 tical photographic results obtainable. 



It was shown theoretically by J. J. Thomson and confirmed ex- 

 perimentally by Whiddington [1912] that the loss of velocity of cathode 



