PHYSICAL ASPECTS OF IMAGE FORMATION 



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will provide the conventional difTraction disk (Airy's disk). Imparting 

 a to-and-fro motion to the focusing button discloses a small streak 

 of light, pointing to the field centre, and another one, at right angles 



Fig. 1.25. Field curvature. 



to the former: the objective is exhibiting astigmatism. These two small 

 right-hand segments, 90° from one another and lying in two different 

 focusing planes, are astigmatic focal lines: they are mainly seen at 

 the field edges. Figure 1.26 shows such a focal line. The diffraction 

 fringes surrounding it are feeble and virtually invisible in a microscope. 



Fig. 1.26. Diffraction fringes surrounding a focal line (astigmatism). 



The geometrical aspect of the problem prevails here. Both spherical 

 aberration and coma must be considerable in order to cause the 

 geometrical aspect of the phenomena to be evinced. This is the reason 



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