824 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



/8, and if h is so large that p > dj then somewhere along the side of the 

 profile there should be a very sharp length of shadow-edge. In Fig. 3 the 

 sharpest length of shadow-edge is neither at top nor at bottom of the side, 

 but in an intermediate position. 



In repUcas other than those of particles which are dispersed on flat sub- 

 strate surfaces, the topography of the surface in the shadow region is usu- 

 ally not planar. In general the form of the shadow-edge is a compHcated 

 function of n, of the finite source, and of the profile casting the shadow. Its 

 intrinsic resolution may vary from large values down to nearly zero. This 

 last fact is important since it implies that the intrinsic resolution of the 

 sharpest portions of shadow-edges may often he assumed to be so small that the 

 observed resolution is that of the microscope itself. These remarks also apply 

 to shadow profiles, for which the special conditions required for extreme 

 sharpness probably occur more frequently than for shadow-edges. 



d) Total Resolution 



The intrinsic resolution of the film is only part of the total resolution, 

 which includes also the resolution of the electron microscope. This is de- 

 termined theoretically by the wave-length of the electrons (about O.OSA) 

 and the numerical aperture of the microscope (about 0.003) as modified by 

 lens aberrations. The theoretical value of 10 A is seldom attained in practice 

 because of lens imperfections associated with use of the microscope. Prac- 

 tically, microscope resolution usually lies in the range 30-50 A, for the RCA 

 type EMU microscope with which the micrographs were taken. In the 

 X 30,000 enlargements shown, d>^ A becomes 10"^ cm, just the limit of 

 resolution of the eye. In general the resolution of the micrographs is not 

 this good, in part because the intrinsic resolution is larger but also because 

 it is necessary to find suitable fine-scale features to test the resolution. 



