890 
CAPTAIN ABNEY AND LIEUTENANT-COLONEL FESTING 
furnishes almost entirely a line spectrum and, in fact, our measurements were referred 
to an empiric scale derived from the absorptions of this body, before we were 
fortunate enough to obtain a wave-length scale from strict comparison with the solar 
spectrum. We believe that the chloroform spectrum might be almost better to give 
a fiducial scale. The multiplicity of fine lines in both of these substances makes the 
mapping the spectra comparatively easy when once the position of those lines has 
been accurately fixed. 
Source of light .—In the positive pole of the electric light we have a source of light 
which may be considered of uniform brightness, if the light emitted from the crater 
only be employed. Whether the crater be or h, inch in diameter (the one being 
produced by a small magneto-electric machine, and the latter by a larger one) the 
brightness remains constant, being apparently that due to the temperature at which 
carbon is vaporized. If then an image of the crater be projected on the slit it 
is evident that the spectrum produced will be a continuous one (crossed, perhaps, in 
the more refrangible regions by bright lines) and of constant brightness. It is worthy 
of remark that in no case have we found the continuous spectrum in the least refran¬ 
gible region to have any signs of bright or dark lines when the crater is projected on 
the slit. We discuss further on the possibility of any of the lines mapped being due 
to the carbon vapour in the arc. In order to get as large a portion as possible of the 
crater available for projection on the slit, the lower pole (which in this case was the 
positive) was placed slightly behind the upper pole. The electric light we employed 
was generated by an “M.” Gramme machine, driven by a small three horse-power 
Brotherhood engine, of whose performance we cannot speak too highly. The current 
is of such a tension that it is capable of supporting an arc of some ^-inch in length. 
The lamp employed was a hand-lamp of the Sautter Lemonier's pattern, and for 
general photographic spectrum work is everything that can be desired. For photo¬ 
graphic purposes the radiation emitted from a gas flame is much too feeble to be of 
any service, and there is no economy in using the oxy-hydrogen light when the 
electric light is available. 
Tubes for holding the liquids .—Tubes of 2 feet, 6 inches, and 3 inches respectively 
were used for the more common and inexpensive liquids, but for those which were more 
difficult to obtain and costly the two shorter tubes were invariably used. The longer 
tubes had diameters of 2 or 1^ inches, and the shorter of f-inch and ijrinch. In order 
to get a bright image on the slit, and at the same time to cover the whole aperture, 
the carbon points were placed about 3 feet from the collimator. The diameter of the 
condensing lens was about 2^ inches, which thus just cast a beam sufficient to fill the 
collimating lens. The smaller tubes were placed in the position where the rays 
crossed, and thus when they were interposed before the slit the brightness suffered 
no diminution except that due to the liquid itself. 
