PROGRESS IN MICROSCOPY 



vision the eye is considered as a perfect instrument provided it functions 

 with a pupil of approximately 05 mm in diameter. Such a diameter 

 does not tally with an eye pupil but with an artificial pupil originated 

 by the microscope itself as happens when the exit pupil of the micro- 

 scope has a diameter of 0-5 nmi (optimum magnification). Then the 

 light entering the eye is restricted to a 0-5 mm diameter circle which 

 acts as pupil: the eye functions as a perfect instrument featured by 

 a single physiological datum: the minimum contrast perceivable. 

 In the case of a small, imaged dark disk, the lowest contrast a normal 

 eye can perceive is approximately 004, for most other objects this 

 figure drops to 002. Owing to diffraction, contrast of the imaged 

 dark disk varies as its radius r. As /• decreases, so does contrast and 

 when y = 004 the image vanishes (for a small dark disk on a white 

 ground). This obtains when the radius /• of the black disk is educed 

 from the following expression: 



013/ 

 r-,^—'^ (1.4) 



For instance, with an objective of numerical aperture ii sin u = 1-30 

 and a mean wave-length value / = 0-6 ^, then r = 003 /n. 



Formula 1.4 implies that r is small in relation to the diffraction disk. 



Image of a small white disk on black ground 



Merely reversing Fig. 1.38 will show the image of a white disk 

 on black ground. The pecked-line curve 1 (Fig. 1.39) shows the 



Aq Bq 



Fig. 1.39. Image of a small while disk on black ground. 



intensity distribution in the geometrical image of the white disk and 

 curve 2 the actual image, taking diffraction phenomena into con- 

 sideration. As the disk with respect to the diffraction pattern is of 



