Tlie Resolution Limit 



57 



IQoi 



4 



! 



# 



1 



..rjmjmosi 



In(H\idiial particles 



Fig. 16. Resolution test photograph taken by AI. von Ardenne, showing 

 gold particles of about 10 A diameter on a very fine membrane 



errors will be discussed in terms of the dimensions of an 

 equivalent object. 



The spherical aberration turns a point into a disk with a 

 diameter 



c?s = C/a^ 



(21) 



where / is the focal length of the objective, a the aperture angle, 

 and C is a dimensionless constant, which depends only on the 

 shape of the electromagnetic field of the objective, not on its 

 dimensions.* 



The diffraction of the de Broglie waves at the aperture, 

 whether limited by a diaphragm or by the divergence of the 

 illuminating beam only, produces an Airy disk with a diameter 



1.22-, equation (14). As the intensity in the Airy disk falls off 



a 



gradually toward the edge, the distance below which two Airy 



* Some authors reserve the name spherical aberration coefficient and 

 the symbol C for the length C/, a regrettable practice, as coefficients 

 should be always dimensionless. 



