36 



PROGRESS IN MICROSCOPY 



the limit of perception. Such is the case when 



002 / 



£ = 



2/7 sin z/ 



(1.5) 



For instance, with a 1-30 N.A. objective and / = 0-6 /<, then 

 £ = 005 i-i. Naturally, as in the previous example, this theoretical 

 result applies only to an aberration-free instrument. 



Luminous thin streak on black ground imaged in incoherent illumination 



This streak is educed at once from the image of the thin, black line. 

 The distribution of light-intensities in the image of a thin streak of 

 light on a black ground is shown by curve 2 in Fig. 1.41. There are 



Fig. 1.41. Luminous thin line on black ground imaged in incoherent illumination. 



no zero minima in the diffraction pattern. As with the small luminous 

 disk on black ground, the problem of descrying a thin streak of light 

 on a black ground is merely a matter of luminous flux; to be perceived, 

 a very thin streak of light must emit enough of it. 



Area edge imaged in incoherent illumination 



Let us consider as object an illuminated half-plane, separated 

 from the black area by a straight line (Fig. 1.42). The pecked curve 1 

 (Fig. 1.43) shows the structure of the geometrical image along a parallel 



Fig. 1.42. Illuminated half-plane object. 



