94 H. A. Rowland—UConcave Gratings for Optical Purposes. 
and so by micrometric measurements the relative wave-lengths 
are readily determined. Hence, rowing the absolute wave- 
length of one line, the whole spectrum can be measured. Pro- 
fessor Peirce has determined the absolute wave- Tatigek of one 
line with great care and I am now measuring the coincidences. 
This method is greatly more accurate than any hitherto known, 
as by a mere eye inspection, the relative wave-length can often 
be judged to i part in 20,000 and with a micrometer to 1 in 
1,000,000. Again, in dealing with the invisible portion of the 
spectrum, the focus can be obtaine by examining the super- 
imposed spectrum. Captain Abney, by using a concave mirror 
in the place of telescopes, has been enabled to use this method 
for obtaining the focus in photographing the ultra red rays of 
the spectrum. It is also to be noted that this theorem of the 
normal spectrum applies also to the flat grating used with 
telescopes and to either reflecting or transmitting gratings; but 
in these cases only a small portion of the spectrum can be use 
as no lens can be made perfectly achromatic. And so, as the 
distance of the micrometer bas constantly to be changed when 
one passes along the spectrum, its constant does not remain 
constant but varies in an irregular manner. But it would be 
possible to fix the grating, one objective and the camera rigidly 
on a bar, and then focus by moving the slit or the other objec- 
tive. In this case the spectrum would be rigidly normal, but 
would probably be in focus for only a small length and the 
adjustment of the focus would not be automatic. 
ut nothing can exceed the beauty and simplicity of the 
concave grating when mounted on a movable bar such as I 
haye described and illustrated in Fig. 1. Having selected the 
grating which we wish to use, we ecpiren it in its plate- neh 
and put the proper collimating eye-piece in place. Wet 
carefully adjust the focus by altering pie length of D until ee 
cross-hairs are at the exact center of curvature of the grating. 
On moving the bar the whole series of spectra are then in exact 
focus, and the value of a division of the micrometer is a known 
quantity for that goto grating, The wooden wa 
the distance AC. We can thus set the instrument to any par- 
ticular wave-length we may wish to study, or even determine 
the wave-length to at least one part in five thousand by a sim- 
ple reading. ~ By having a variety of scales, one for each s = 
trum, we can immediately see what lines are superimpos 
each other and identify them accordingly when we are peasant 
ing their relative wave-length. On now replacing the eye-piece 
by a camera, we are in a position to photograph the spectrum 
with the greatest ease. We put in the sensitive plate, either 
