38 J. M. Blake—Notes on Diffraction Gratings. 
the error due to an increment in the distance of the lines, 
in passing from one side of the grating to the other, is three 
times as great in the Nobert ruling. In each case the ban 
come straight in general direction, when one of each biahe of 
gratings is rotated in the plane of the ruling 180 degrees. 
Since writing the above, with the exception of a revision of 
the part relating to the photographing of the bands developed 
by superposition, I find a paper on this same subject by Lord 
Rayleigh, in Phil. Mag. for Feb., 1874. He has es tip 
directions of the pert lines.” And suggests “the iregu- 
e bars, due to imperfection in the ruling when 
met ae is very closely approached, might be made useful 
as a test.” He speaks also of irregular bands seen on individ- 
ual photographs, and says that “the disappearance of the first 
spectrum is very unusual; but that it is common for bands to 
appear in the fourth and higher s spectra.” 
His explanation of the cause originating these bands is dif- 
ferent from mine. He says “when examined under the micro- 
scope, the opaque bar on the copy, which corresponds to the 
shadow of the groove (ruled line) of the original, is seen to be 
composite; being not unfrequently traversed along its length 
by several fine lines of transparency.” And again, “in the 
process of copying: the groove of the original is widened into a 
bar, whose width depends on closeness of contact, an element 
which necessarily varies in different parts of the plate.” In 
regard to the definition of a 3000 line grating by Nobert, he 
appears to show that the Nobert ruling which he examined 
also oe eurved lines, and that this curvature injured de:inition. 
e copies “not perceptibly inferior to — bok aan 
— with copies of the 3000 line grating, with a 
g power, could “make out nearly, but not crite; all that is 
at in Angstrém’s ma 
age later paper in the March number of the same journal, 
he speaks of the on of a 6000 line Nobert ruling to 
the one of 3000 lines by the same maker. It appears to me 
that an important point which is apparently pe should 
be borne in mind in the manufacture of gratings; since we 
have here an indication, thar with 6000 lines to the inch, the 
maximum of cai a abd has been — for me reason 
PeeWee SN Ss EAE a 
