PHYSICAL ASPECTS OF IMAGE FORMATION 49 



1-22 A/(2/2sinw). Two pin-point details on black ground, emitting identi- 

 cal luminous fluxes will appear discrete only if the relevant diffraction 

 disks are not too close to one another. Let us first assume that the 

 two pin-point details act as would two incoherent sources on black 

 ground. In Fig. 1.63 the distance between the two diffraction disks 



Fig. 1.63. Diffraction patterns the distance of which is greater than their diameter, 



amounts to approximately twice their diameter and are readily dis- 

 joined. Resolving power of the two disks is usually considered to be 

 reached when the central maximum of one of the diffraction patterns 



Fig. 1.64. Diffraction curves as resolving power is attained (incoherent illumination). 



corresponds to the first zero minimum of the other and conversely, 

 i.e. when the diffraction curves are as shown in Fig. 1.64. Under 

 such conditions, light-intensity, half-way between the two images, 



