624 Transactions of the Society, 



Instead of going into such details, I will deal with other 

 features of the images under consideration, most of which have at 

 one time or another been advanced as insuperable obstacles to the 

 validity of the diffraction theory. 



The first came from Mr. Gordon, who seems to have misunder- 

 stood one of the published accounts of the theory in such a way 

 as to lead him to claim that, according to Prof. Abbe, the image 

 should stand still when the object is moved, and he thus appeared 

 to have succeeded in bringing the theory under the ban of the 

 famous " which is absurd " of our old friend or foe, Euclid. The 

 late Prof. Everett turned the tables on Mr. Gordon in what was 

 probably his last paper, and I fancy that Mr. Gordon's astute 

 rejoinder to that paper failed to impress either those present at 

 the meeting or those who read the report. However, my method 

 of treating the problem supplies an alternative proof that the 

 diffraction-image must move conformably to any movement of 

 the object. For I have shown that the diffraction-spectra must 

 always produce an image of a simple plane grating coincident 

 with the geometrical image, and, as the latter obviously moves in 

 proportion to movements of the object, the diffraction -image must 

 do the same. Competent physicists will see at once that my proof 

 is really identical with Prof. Everett's, as it relies on the same 

 theorems ; it, however, has the advantage of avoiding the inter- 

 mediate step which Mr. Gordon made his sheet anchor for objec- 

 tion, and to thus be free from that objection ; and it may therefore 

 be acceptable to those who do not care to expend their mental 

 energy in going through mathematical investigations. 



The next objection also came from Mr. Gordon. He claims 

 that there is nothing in the diffraction theory to show how 

 the right number, the right shape and length, and any existing 

 irregularity of the lines, come to be correctly shown in the 

 image. Mr. Gordon's objection would seem to be prompted by 

 a complete misunderstanding of the diffraction theory, for he 

 persistently speaks of the lines seen in the image as diffrac- 

 tion images of the source of light, although it must be obvious 

 that it is physically impossible that such images should be 

 formed in the plane of the final image ; I can only think that 

 Mr. Gordon confuses the diffraction spectra formed in the upper 

 'principal focal plane of the objective, in the form of small images 

 of the source of light, with the diffraction pattern resulting from 

 the interference of the waves concentrated in these images when 

 they have again spread out and arrive in the plane of the micro- 

 scopical image of the object. I am reluctantly driven to this 

 explanation, for how could Mr. Gordon claim as an objection to 

 the Abbe theory the fact that, with a candle as the source of light, 

 the image of a grating as seen through the eye-piece does not 



