31 



ON MR. GLAZEBROOK'S PAPER ON THE ABERRATION OF 

 CONCAVE GRATINGS 



[American Journal of Science [3], XXVI, 214, 1883 ; Philosophical Magazine [5], 



XVI, 210, 1883] 



In the June number of the Philosophical Magazine., Mr. R. T. Glaze- 

 brook has considered the aberration of the concave grating and arrives 

 at the conclusion that the ones which I have hitherto made are too 

 wide for their radius of curvature. As I had published nothing but a 

 preliminary notice of the grating at that time, Mr. Glazebrook had not 

 then seen my paper on the subject, of which I gave an abstract at the 

 London Physical Society in November last. In this paper I arrive at 

 the conclusion that there is practically no aberration and that in this 

 respect there is nothing further to be desired. 



The reason of this discrepancy is not far to seek. Mr. Glazebrook 

 assumes that the spaces are equal on the arc of the circle. But I do 

 not rule them in this manner; but the equal spaces are equal along 

 the chord of the arc. Again, the surface is not cylindrical, but spherical. 



These two errors entirely destroy the value of the paper as far as my 

 gratings are concerned, for it only applies to a theoretical grating, ruled 

 in an entirely different manner from my own, and on a different form 

 of surface. 



I am very much surprised to see the method given near the end of 

 the paper for constructing aplanatic gratings on any surface, for this 

 is the method by which I discovered the concave grating originally, and 

 the figure is the same as I put on the blackboard at the meeting of the 

 Physical Society in November last. I say I am surprised, for Mr. Glaze- 

 brook's paper was read at the Physical Society, where I had given the 

 same method a few months before, and yet it passed without comment. 

 Indeed, I have given the same method many times at various scientific 

 societies of my own country. However, as Mr. Glazebrook was not 

 present at the meeting referred to, he is entirely without blame in the 

 matter. 



