736 PROCEEDINGS OF THE SOCIETY. 



the meaning attached to it by me — does indeed represent a periodical 

 disturbance, but of an unprecedented kind. Owing to sin 2 a being 

 always positive, with a period of it, the disturbances in the aether are 

 all on the same side of the ray, with a period equal to half that of 

 the light-waves causing this novel phenomenon ; and, stranger still, the 

 amplitude is a constant, 2 C ; the influence of the width of the slit 

 has therefore disappeared, or, in other words, slits of all widths cause 

 the same disturbance, which, however, is not light. Other interpretations 

 may be possible by making different assumptions as to the significance 

 of Mr. Gordon's a, but if a were to be put forward as an angle not 

 depending on time, say as my (3, then the expression 2 C sin 2 a would 

 no longer represent any undulations of any kind, and would, therefore, 

 become hopelessly absurd, as a permanent dislocation in the aether is 

 quite unthinkable. 



A large part of the preamble of Mr. Gordon's note falls to the 

 ground through this proof that the error in the matter is on his side. 



It remains to briefly deal with Mr. Gordon's reference to Prof. 

 Everett's paper. 



I am truly glad to see that Mr. Gordon now admits the value and 

 validity of Prof. Everett's proof of the conjugate movements of object 

 and image according to the method of treating the formation of micro- 

 scopical images which Prof. Abbe has introduced. But that is pre- 

 cisely what I claimed in my paper and referred to, more particularly in 

 the eleventh line on page 624. Mr. Gordon's attempt to prove me at 

 variance with Prof. Everett is, therefore, completely unfounded. The 

 changes in phase relations which take place when a grating is moved 

 across the field of vision, and which Prof. Everett used in his elegant 

 proof, have no connection whatever with the phase-law which I have 

 established as holding for the light emanating from any one slit of a 

 stationary grating, and which is due to interference of the light from 

 different parts of each individual slit. 



The further question raised by Mr. Gordon, as to whether my appli- 

 cation of the well-established law of equal optical paths between geo- 

 metrically conjugate points is reconcilable with the Abbe theory, arises 

 from Mr. Gordon's views as to the scope and significance of that theory, 

 which I do not share. I think the reasoning which I have given in 

 my paper will appear both clear and conclusive to the majority of readers, 

 and I do not consider it necessary to add to it. 



