XII, ELECTRON MICROSCOPY 383 



produced in discharge tubes or obtained by applying a high positive 

 potential to an electrode near the tungsten filament heated to incan- 

 descence consisted of corpuscles of negative electricity known as 

 electrons. It was also known that those electrons could be deflected 

 by a transverse magnetic or electrostatic field and that the electric 

 fields produced between coaxial cylinders or the magnetic field 

 produced by a current passing through a coil of wire wound coaxially 

 with the electron stream would concentrate the beam into a small 

 point. While the literature contained some casual remarks concern- 

 ing the striking similarity between the behavior of the cathode ray 

 streams in such magnetic or electrostatic fields and rays of light 

 traveling through a lens system, the real significance of the similarity 

 was not realized until some years later. 



In 1924, however, de Broglie predicted theoretically that a moving 

 electron should have a periodicity associated with it. This meant 

 that there could be assigned to a moving electron a wavelength 

 having a value given by the formula X = (150/T^)'^', where X is the 

 Avavelength in Angstroms (10~* cm.) and V is a measure, in volts, of 

 the velocity of the electron. Thus, electrons accelerated by 60,000 

 V. will have a wavelength of 0.05 A., which is only 1/100, 000th that 

 of visible light. This idea was developed theoretically and became 

 the basis of our modern quantum mechanics. The correctness of 

 de Broglie's theory was demonstrated experimentally by G. P. 

 Thompson and by Davidson and Germer around 1927. These 

 workers also measured the wavelength of the electron stream and 

 showed that it agreed with de Broglie's calculations. 



Almost simultaneously with these developments, H. Busch was 

 working on the action of a rotationally symmetrical magnetic field 

 on an electron beam traveling along the axis of symmetry and in 

 1926 and 1927 published papers demonstrating that such magnetic 

 or electrostatic fields acted as true lenses, i.e., they were capable 

 of transforming a beam of electrons diverging from a point on the 

 axis of symmetry of the system into a beam converging to or diverging 

 from a second point on the same axis. He also showed that one could 

 define focal lengths for such lenses and that they could be expressed 

 in a form identical with that already in use for optical lenses. 



In pointing out that a rotationally symmetrical electric or magnetic 

 field was a true electronic lens, Busch initiated the science of electron 

 optics. Very shortly afterward other workers showed theoretically 

 that the analogy between electron optics and light optics was quite 



