26 J. RHEINBERG ON AN OVERLOOKED POINT CONCERNING 



the resolution of the two clear lines is compared with the 

 resolution of a band of clear lines. The explanation of the 

 resolution of two dark lines as compared with a band of dark 

 lines — as in the case of the Grayson ruling experiment — is 

 analogous. The reason for treating the former case rather than 

 the latter is that it is simpler to explain, and it illustrates the 

 principle just as well. 



It is a matter worthy of particular note that the new fact 

 concerning resolution was not first discovered and then explained. 

 In this case Dr. Johnstone Stoney first reasoned the matter out 

 many years ago and then made the experiment, testing the 

 result when Grayson's rulings became available. It is a striking 

 example of the way in which the Abbe diffraction theory may be 

 turned to account. 



A further point is this : — According to theory, the distance 

 apart of the lines in the image must vary inversely as the dis- 

 tance apart of the maxima in the intensity Curve. But when we 

 cover up the greater portion of the lateral maxima in the curve 

 b, the distance between what remains (x, Fig. 6) is less than 

 the distance between their brightest portions had they been 

 uncovered (y, Fig. 6). Therefore we ought to expect the lines 

 to appear farther apart in the first case than in the last. And 

 this may, under favourable conditions, be shown experimentally 



There are certain interesting problems which it seems to me 

 the newly observed fact may materially assist to clear up. Not 

 the least of these is the question as to how far one part of an 

 object influences the formation of the image of another part. 

 This is a question on which differences of opinion still exist 

 amongst those who have occupied themselves with the problems 

 of microscopic optics, and one which is of great importance because 

 it assists in determining to what degree of approximation the 

 image of an object may be made to faithfully depict the structure 

 of the object. 



It may be in place to mention that the only difference between my 

 arrangement of the experiments and those of Dr. Johnstone Stoney is 

 that he arranged for showing them with oblique light, using two maxima 

 only, whilst I have preferred to make them with axial light, using three 

 maxima. 



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