210 On the Aberration of Concave Gratings. 
The E line has components about * ■ of the wave-length 
apart. I believe I can resolve lines much closer than this, 
say 1 in 100,000 at least. Hence the idea of a limit has not 
yet been proved. 
However, as some lines of the spectrum are wider than 
others, we should not expect any definite limit to resolving 
power, but a gradual falling-off as we increase our power. At 
first, in the short wave-lengths at least, the number of lines is 
nearly proportional to the resolving-power ; but this law 
should fail as we approached the limit. 
XXX. On Mr. Glazebrook' s Paper on the Aberration of Con- 
cave Gratings. By Henky A. Rowland, Professor of Phy- 
sics, Johns Hopkins University, Baltimore* . 
IN the June number of the Philosophical Magazine Mr. R. 
T. Glazebrook 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 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 theo- 
retical grating ruled in an entirely different manner and on a 
different form of surface from my own. 
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 con- 
cave grating originally, and the figure is the same as that I 
put on the black board 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 at 
many of our own scientific societies. However, as Mr. Glaze- 
brook was not present at the meeting referred to, he is entirely 
without blame in the matter. 
* Communicated by the Author. 
