378 Mr. E. T. Glazebrook on 
A B C in B. Then, if P be any point in A B C, Q P + P Q' is a 
minimum when P coincides with B ; and the portion of the 
line ABC which is effective in illuminating Q' is an element 
near B*. Similar results follow for each of the other reflecting - 
lines of the grating; and these small elements all lie in the 
plane Q Q'. Thus, in considering the diffraction effects, we 
need only consider a section of the grating by this plane; and 
this is what has been done both by Professor Rowland and 
myself. 
With regard to the last paragraph of Prof. Rowland's note, 
I would avail myself of this opportunity of stating that I had 
no idea that the method referred to was that by which he had 
arrived at the theory of the concave grating, or that any 
account of such method had been previously given. It seemed 
to me instructive, and so I mentioned it. Prof. Rowland's 
paper is a conclusive proof of its power. I can only express 
the regret that my absence from the meeting of the Physical 
Society in November, at which Prof. Rowland's paper was 
read, has led me to appear to claim a priority which really 
belongs to him. 
In Prof. Rowland's paper he treats of the aberration by 
considering how much the spaces between two consecutive 
lines ruled near the edge of the grating differ from theoretical 
perfection. In mine I determine the difference in phase be- 
tween the light to the focus coming from the two extreme 
lines of the grating. 
With the notation and figure of my previous paper, Q (fig. 2) 
Fig. 2. 
is the source of light, Q x the focus, the centre of the sphere, 
* Verdet, Optique physique, tome i. § 58. 
