MAGNETIC OBJECTIVE 349 



polished inner surface. If the apertures are too small, large errors are 

 introduced by reflection of electrons at the side walls of the boring. 



Magnetic Objective 



In Fig. VIII-24 is shown an object stage POP, placed just above the 

 pole pieces of the magnetic objective coil. The pole pieces have a 

 clearance of 10 mm. A brass disk, with a boring of 0.1-mm radius, acts 

 as a diaphragm stop. The incident electrons passing through the 

 specimen are scattered through various angles. It is only those elec- 

 trons that pass through the specimen without appreciable change in 

 path that can be considered as contributing anything to the first-stage 

 electron image, which is formed by the transmitted beam. 



The angular aperture of the incident beam is 2a c , or 2a' c when it attains 

 its maximum value. The electrons leaving the specimen below P that 

 contribute anything to the first-stage image must be contained in the 

 angular aperture 2a a defined by the resolving power of the magnetic 

 objective, namely 



2N.A. 2a a _j 

 R.P. = = — cm 1 



X A 



Without a magnetic field below the plane POP to deflect the emitted 

 electron beam, a a = a c . The magnetic field of the objective coil con- 

 stricts the electron beam emitted at P so that a beam of larger angular 

 aperture 



oc a = «c + «6 



is made available for image formation. Under these conditions a& is 

 the increase in the angular aperture due to the magnetic field of the ob- 

 jective coil. 



Thus the density of the electrons available for image formation, at a 

 given incident electron velocity, depends on the forward scattering prop- 

 erties of the specimen and on the magnetic field strength of the magnetic 

 objective. 



If a resolution of 10 -6 cm is desired with 60-kilovolt electrons (X = 

 4.88 X 10 -10 cm), the angular aperture 2a a should be about 10 -3 . 

 Under practical conditions the aperture of the diaphragm of the objec- 

 tive coil is made 10 times larger, comparable to a diaphragm stop having 

 a radius of about 0.1 mm. 



The very narrow beam of electrons leaving P in the plane of the object 

 is deviated as it passes through the magnetic field of the objective coil 

 to such an extent that it crosses over before it is brought to a focus in the 



