36 J. M. Blake— Notes on Diffraction Gratings. 
difficult to prevent the doubling and splitting up of the bands 
at one end, where the light that “entered the lens became more 
aa to the grating, “and the irregular bands of the indi- 
twenty it solar, was tried, ‘and then by using sun- light a 
much better result was secured. It was found altogether best 
with the 6480 line grating to move camera and grating at- 
tached, so as to make an angle with the direct beam, and into 
the proper direction to have the blue and violet of the first 
spectrum cover the image. Then the bands came out with a 
distinctness not before obtained, and the actinic effect was suf- 
ficiently uniform over the whole surface. The resulting photo- 
graph was transferred to the wood block for engraving, and 
the result is given in fig. 1 
The same process was sso Aare with the 2 2000 8 ane ex- 
cepting that it was found best to bring the image into the 
given in fig. 3; but unfortunately 
the minute irregularities so well shown 
in the photograph could not be mee 
dueed in the wood cut. e first 
spectrum was impaired to some extent 
in these particular gratings, which 
were probably the third reproduction 
from the original. The splitting up 
actinic illumination at the two ends 
seemed unavoidable. Next inicro-photograpks were tried with 
the same illumination. The result with the Nobert grating did 
not possess sufficient interest to reproduce here. The original 
lines were shown. The Rutherfurd grating gave the result repre- 
* 
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