PHYSICAL ASPECTS OF IMAGE FORMATION 



culated. Substituting Z for 383 gives, 



Cn=r 



znsmu 



(1.2) 



As a matter of fact, it is the central disk which acts as the 

 imaged point A. The luminous rings surrounding the central disk 

 merely enlarge it and are spurious light. 



It follows that enlarging the image is always detrimental and, 

 to observe very fine details, the diameter of the central spot is to be 

 as minute as possible. This result is achieved, in accordance with 

 equation 1.2, by using a high numerical aperture objective. Let us 

 take for example, a N. A. ns'mu = 030 with magnification g = 25. 

 At wave-length / = 06 microns, Co = 30 microns. Phenomena can 

 be considered in the specimen plane, whereupon they occur as if the 

 pinpoint object A were substituted for a small disk having 

 C^lg = 1-22 Klilnsinu) as radius. With a 1-30 N. A. objective the diam- 

 eter of the central diffraction disk drops to 028 microns for the same 

 v/ave-length. In the specimen plane, this diameter depends solely on 

 the objective N. A. and the wave-length of the light used. 



Figure 1.6 shows the diffraction patterns of various numerical 

 apertures for the same intensity in the disk centres. 



^ 



Iaa y' 



J ^o« 

 n Sin u = I 30 



FiCi. 1.6. Diffraction patterns with various numerical apertures in the specimen plane 

 (same intensity in the disk centres) {?. = 0-6//). 



However, it should be mentioned that the foregoing applies strictly 

 to low or medium numerical apertures only. When high numerical 

 apertures are involved, A. Wolf has shown that the disk is somewhat 



