

152 Notes. 



objection to my theorem in the face of this striking, and indeed 

 startling, experiment. 



While experimenting in Mr. Itheinberg's laboratory, and by 

 the exercise of some patience and discrimination, we were able to 

 confirm theory in yet another and equally convincing manner by 

 means of the same grating. It is a simple deduction from my 

 theory, that if by any process we could reverse the phase relation of 

 the second spectrum to the direct light and to the first spectrum, 

 the relative width of the lines should be shown the reverse of truth, 

 i.e. the narrow lines should appear broadened out, whilst the broad 

 ones should be sharpened up. By careful use of the " compen- 

 sator " we were able to produce this effect also ; but, as in the present 

 form of the compensator the experiment is a delicate one, and the 

 proper conditions easily upset, I defer the demonstration for a 

 future occasion, as I am naturally anxious not to provide any 

 opportunities for the misrepresentation of facts or the raising of 

 false issues, such as would be afforded by disturbing the theoretically 

 necessary adjustments. 



In conclusion, I might point out that I have recently discovered 

 that phase-reversals of the kind which I have dealt with in my 

 paper, and the importance of which for the full explanation of 

 microscopical images has been there insisted upon, have been found 

 many years ago by Sir George Airy and by Schwerd, in connection 

 with numerical determinations of the amplitudes of diffracted 

 light ; but as, from their point of view, the phase of the diffracted, 

 light was immaterial — the intensity being the quantity sought — no 

 importance was attached to it, and no attention called to this 

 interesting fact, which would, indeed, seem to have been com- 

 pletely lost sight of until now. 



The Influence on Images of Gratings of Phase-Differences 

 amongst their Spectra. 



By Julius Eheinbeeg. 



Plate IV. 



Tiie accompanying photographs, taken with the Abbe Demonstra- 

 tion Microscope, afford some experimental evidence of the results 

 brought out by Mr. A. E. Conrady's development of the theory of 

 microscopic vision. 



The following is, I think, a simple method of regarding those 

 results, which will assist in understanding the photographs. 



