20 PROGRESS IN MICROSCOPY 



the path difference J at any point of the wave is merely a function 

 of the distance to the axis of said point. Spherical aberration is present 

 in the microscope objective. Such aberration is usually pretty well 

 corrected and maximum values of the A deformation always small. 

 In microscopy, the geometrical aspect of the phenomenon, i.e. de- 

 velopment of a caustic curve, never occurs. With regard to defective 

 focusing, it may be said, in general, that spherical aberration decreases 

 the central peak of the diffraction pattern and eliminates the black 

 rings as in Fig. 1.10. Provided the surface of the actual wave 2",' be 

 enclosed between two spheres one quarter wave-length apart, it is 

 considered that the diffraction-pattern structure does not differ ma- 

 terially of the Airy disk. This is Rayleigh's law, which serves to calcul- 

 ate objectives. The diffraction theory enables one to calculate structure 



007 008 



Fig. 1.22. Isophotes in a meridional plane in presence of spherical aberration (after 



J. Picht). 



images in various planes parallel to the plane .t by combining spherical 

 aberration and defective focusing. The phenomenon being one of revol- 

 ution, the results can be shown by plotting the lines of equal intensity 

 as in the case of Fig. 1.14. Figure 1.22 shows the result achieved when 

 spherical aberration has a .1 peak value not exceeding 2/. This time, 

 obviously, the symmetry in relation to any plane perpendicular to AA'^, 

 has vanished. If the best focusing plane is considered to be the plane ti, 



