52 



PROGRESS IN MICROSCOPY 



circular shape. Conversely, in coherent illumination (Fig. 1,70) there 

 is some interference between the various diffraction disks. The rings 

 are completely distorted and even the central disks are no longer 

 circular. This would lead one to believe that the actual shapes of 



Fig. ],68. Image structure according to the objective's N.A. ns'mu = 1-30. 



luminous objects are emerging but such is not the case, since the 

 errors that may be made when interpreting such images can readily 

 be realized. 



Let us now consider the case where two pin-point details act as 

 two coherent sources on black ground. Under the same conditions, 

 resolving power is reached when the distance ;', from the two points 

 in the specimen plane, is given by: 



y 



1 63 A 

 2/7 sin u 



(I. II) 



Two pin-point luminous sources on black ground are not as discrete 

 in coherent illumination as in incoherent. 



H. H. Hopkins investigated how the separation limit of two pin- 

 point sources on black ground varies versus the condenser N.A, The 



