486 PROCEEDINGS OF THE SOCIETY. 



would, on taking out the eye-piece, see a small beam of light in the 

 centre, and on either side of this a certain number of faint spectra. The 

 apparatus of Zeiss consisted of a series of slits in a diaphragm, so spaced 

 that/with a particular size of object-glass suited to the grating, when the 

 series of slits was placed over the object-glass, it would cut out two 

 alternate spectra, and the effect then produced was to double the lines 

 in the image ; and it would be interesting to recall that when Mr. Gordon 

 read his paper he made his demonstration with a similar diaphragm with 

 only two slits, which were so spaced that they excluded those portions 

 of light which the three-slit diaphragm admitted. Prof. Everett's paper 

 was especially interesting because it afforded a mathematical proof of 

 the doubling of the lines, and showed that, provided alternate spectra 

 were excluded, the same phenomenon was produced, so that the experi- 

 ment above-mentioned was not in itself a disproof of the Abbe theory, 

 as the three-slit and also the two-slit diaphragm cut out alternate spectra. 

 The questions which naturally arose were, had this anything to do with 

 microscopic vision ? was the effect produced by the cutting out of the 

 spectra, or was it produced by the slit diaphragms acting as diffraction 

 gratings, causing a doubling of the lines ? It would be perfectly possible 

 to exclude spectra by having a cover glass with small black spots, but 

 would this produce a doubling of the lines ? Prof. Everett's paper 

 seemed to afford an explanation of the reason why Prof. Abbe's theory 

 had been applied with so much enthusiasm to vision in the Microscope. 

 Mr. Gordon showed them that although a certain series of spectra 

 were produced with an illumination of parallel light, as soon as the 

 quality of the light was changed from a parallel to a convergent beam 

 these spectra shifted their positions, and that when the object became 

 practically a radiant source, the light being focussed on the plane of the 

 grating, then these spectra closed up and practically disappeared. When 

 this was the case, it would be impossible to suppose that, as no spectra 

 were now excluded by the three-slit diaphragm in the back focal plane 

 of the object-glass, the Abbe theory could account for the doubling 

 of the lines in the image. As a matter of fact, the doubling of the 

 lines took place whatever the kind of illumination employed, which 

 would appear to show that the Abbe theory was not the cause of the 

 doubling of the lines in the well-known experiment, and that the theory 

 was of purely academic interest, and had no direct bearing of con- 

 sequence on microscopic vision. 



Mr. J. Rheinberg — after reading his note ' On the Influence on 

 Images of Gratings of Phase-Difference amongst their Spectra ' — said 

 he would have liked to discuss the standpoint taken up by Mr. J. W. 

 Gordon and Mr. C. Beck, but time being short he would make his re- 

 marks very brief, and touch on one matter only. It appeared to him 

 futile to argue, as they did, that because certain diffraction effects were 

 obtained by placing diaphragms behind the objective, this fact threw 

 doubt upon the Abbe theory. One might make a hole in a board by 

 drilling it, or produce a hole in a board by a pistol-shot : the presence of 

 the hole produced by the latter method would disprove nothing as to 

 holes produced by drilling operations. Analogously (as he had en- 

 deavoured to show in a previous discussion), although by the inter- 



