10 



PROGRESS IN MICROSCOPY 



travel farther than do those originated by M^. The difference between 

 these two trajectories is the path difference J . Let J„, be the maximum 

 value of A which correspond to the edge of the wave. The depth of 

 focus may be characterised by the path difference J„, . If J,„ = 0, focusing 

 is carried out on the plane of the flawless image whose structure was 

 investigated previously. For a J,„ value other than zero, the focusing 



Fig. 1.9. Path difterence J produced by a shift of the focusing plane. 



plane no longer passes through the flawless image A[^. Assuming the 

 focusing to be carried out at B\ at the short distance .v from .4,',, what 

 would be the structure of the image observed? It follows that the 

 diffraction pattern at B' depends on the defective focusing, i.e. on \,„. 

 However, in general, decreased intensity of the central peak and 

 intensified luminous rings are observed. 



Figure 1.10 shows this. Curve (1) illustrates Airy's disk and 

 curve (2) the diffraction disk structure when focusing is but slightly 

 defective. The diffraction disk is blurred and the image contrasts of 

 extended objects greatly impaired. When out-of-focus becomes marked, 

 the diffraction disk structure is completely altered. Such effects are 

 readily observed in the microscope by using as specimen an object-slide, 

 processed by a vacuum-evaporated aluminium-metallisation and covered 

 by a cover-slip if the objective is of the dry-front lens type. The 

 akiminium coating of such object-slide is never completely opaque: 

 there always remain small holes forming ideal pin-point sources 

 of hght. 



Figure l.Il shows how the first luminous rings are accentuated 

 when defective focusing has a l,„ value smaller than /. When 

 zl,„ = /., Fig. 1. 1 2 shows the minimum to be zero in the centre of the 

 diffraction disk. Light-intensity changes can be evinced by a curve 



