30 



PROGRESS IN MICROSCOPY 



incoherent vibraUons. Let us assume that the numerical aperture 

 ns'mii of the objective is much lower than the condenser's. The central 

 diffraction disk of the objective is wide (curve U) and overspreads 

 in width the diftYaction disks of the condenser at A and B. The in- 

 tensities shown are naturally random. The purpose sought is merely 

 to compare the diameters of the central disks. When points A and B 

 are integral with a small object of high contrast whose size is less 



A 



7 



Fig. 1.35. The diameter of the condenser central diflfraction disk is smaller than 



distance AB. 



than the diameter of the objective's central diffraction disk, the 

 object is seen in incoherent light. Such a case is not met with in 

 microscopy as, in order to have images of high-contrasted objects 

 in incoherent light, the objective's numerical aperture has to be so 

 much lower than the condenser's that, when normal apertures are 

 used in the latter, this is tantamount to using an objective of very low 

 numerical aperture. 



Diffraction disks are of the same magnitude when the N.A. of 

 both objective and condenser are somewhat simikir. Now, since we 

 are considering an object of smaller size than the diameter of the 

 objective's central diffraction disk, this means that points A and B 

 are not to be considered as being as far apart as previously. The 

 points A and B to be selected must be closer so as to be included 

 within the central diffraction disk correlated to the objective. The 

 condenser N.A. selected is usually lower than the objective's. The 

 central diffraction disk of the condenser is thus wider than the objec- 

 tive's and the conditions shown in Fig. 1.34 are reverted to. illumina- 

 tion is virtually coherent. 



Thus far no mention has been made regarding condenser defects. 

 Theoretical considerations show that its aberrations take no effect 

 on image coherence which depends solely on the condenser aperture. 



Other reasons militate in favour of a high-quality condenser: 

 Kohler's method provides a sharp image of the diaphragm D^) oa 



