The Abbe Diffraction Theory. By J. W. Gordon. 371 



By rotating the diaphragm the observer may easily satisfy himself 

 that the number corresponds to the finer ruling-, and that accordingly 

 a doubling of the lines has taken place in one half of the picture. 

 This was, according to theory, to be expected ; for in this case the 

 diffraction image of the wider structure has been artificially made 

 similar to that of the narrower structure. But it could not precisely 

 have been inferred that the image produced in this way would exhibit 

 all the features of a normal Microscope image with entire exactness, 

 and that an incautious observer would certainly take it for the true 

 representation of a structure actually present in the object." 



This last paragraph is further illustrated by the drawing here 

 reproduced as fig. 80, and our author then passes away from the 

 description of experiments to the consideration of the bearing of 

 the Abbe theory upon the examination of fine structures of irregular 

 form, and upon the principles of Microscope construction. Into these 

 fields we are not concerned to follow him. 



Fig. 79. 



Fig. 80. 



Two things are noteworthy in the foregoing description : first, 

 the inaccuracy with which the observed appearances are recorded, 

 and, second, the limited nature of the examination actually made. 

 As to the first. In figs. 74 and 80 sketches are given of the appearance 

 in the Microscope of the ruling under different conditions of observa- 

 tion, and both the drawings and the text convey the impression that 

 no difference is to be detected under these conditions between the 

 image of the coarse ruling and that of the fine. The Abbe theory 

 itself favours this view ; for if the distinctive features of the picture 

 are derived from the distinctive diffraction spectra, it seems natural 

 to infer that when the distinctive diffraction spectra are shut out no 

 difference will, in fact, be discernible between the images. The fact 

 is not so. In both these figures, if correctly drawn, the upper half — 

 corresponding to the coarse ruling — would be only half as bright as 

 the lower half. It may be said that the argument holds even with 

 the necessary correction of the data, but I will not stay to discuss 



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