476 A. E. CONRADY ON RESOLUTION WITH 



further important and easily verified result of Abbe's theory is 

 that with any properly constructed objective these diffraction 

 spectra appear at perfectly equal distances from each other, no 

 matter how they are caused to travel about in the clear aperture 

 (by manipulating an oblique light stop, for instance), and that 

 with any given objective their distance apart is directly pro- 

 portional to the number of lines per inch of the structure causing 

 them. 



Bearing these facts in mind we compare the results obtainable 



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Fi-. 1. 



with dark-ground illumination with the limit theoretically at- 

 tainable with direct light, i.e. when a ray of direct light just 

 enters the clear aperture on one side, and the corresponding ray 

 of the first difiraction spectrum at the opposite side as is shown 

 in fig. 1, where the small circle represents the full aperture of 

 an objective, h being the direct light, and c the first difiraction 

 spectrum. 



To procee 1 to dark-ground illumination, we will imagine that 

 our objective is capable of being opened out to the aperture of 



