50 



PROGRESS IN MICROSCOPY 



amounts to 0-74, viz. a loss of 26 per cent. Figure 1.65 shows the two 

 diffraction disks as resolving power is attained. From expression (1.2) 

 the distance between the two luminous points in the specimen plane 

 is given by the expression: 



1-22/1 

 .y=;^.~- (1.10) 



As the N.A. of the microscope's objective increases the diameter 

 of the diffraction disk shrinks and the closer details are resolvable. 

 Figures 1.66, 1.67 and 1.68 show how an image structure evolves 



Fig. 1 .65. Two diffraction patterns as resolving power is attained (incoherent illumination). 



according to the objective's N.A. Tn Fig. 1.66 the N.A. is inadequate 

 and details cannot be resolved: the image is indifferently defined and 

 surrounded by diffraction fringes. As N.A. increases (Fig. 1.67) the 

 structure outline of the object emerges. As N.A. is increased (Fig. 1.68) 

 the diffraction disks are sufficiently fine to show that the object con- 

 sists of discrete luminous points. This evinces clearly the significant 

 effects of diffraction on the image structure of small objects. Figures 1 .69 

 and 1.70 are instructive, too: they show the aspect of an object con- 

 sisting of luminous points on dark ground in coherent and incoherent 

 illumination. The imaged diffraction disks of the luminous points add 

 up in incoherent illumination (Fig. 1.69). Diffraction rings retain their 



