DR. A. SCHUSTER ON THE SPECTRA OF METALLOIDS. 
39 
increase, but by a decrease, in the strength of the spark. I shall prove that, at the 
lowest temperature at which oxygen is luminous, it gives a continuous spectrum, 
which disappears as the temperature is increased. It is not easy to see how this can 
be due to any other cause than the one which I have advocated.* 
Channelled space spectra .—Standing between line spectra and continuous spectra, 
we have, as I already mentioned, spectra of fluted bands or channelled spaces. Such 
spectra, when seen in spectroscopes of small dispersive powers, appear, as a rule, made 
up of bands, which have a sharp boundary on one side, and gradually fade away 
on the other. When seen with a more perfect instrument, each band seems to be 
made up of a number of lines of nearly equal intensity, which gradually come nearer 
and nearer together as we approach the sharp edge, and finally fuse together. The 
sharp edge is the limit toward which the lines gradually approach. This edge is 
sometimes towards the red and sometimes towards the blue end of the spectrum. 
There is another class of channelled space spectra, which, though never accurately 
described or measured, is, as I. have reason to believe, by no means uncommon. These 
spectra appear, in instruments of low dispersive power, as made up of bands covering 
* Since the above was written, Professor Stokes has suggested to me a way by which the continuous 
spectrum might be produced, and which differs from the one described above as that of molecular 
impacts. 
We have reason to believe that the mere motion of matter through the ether is insufficient to pi’oduce 
vibrations. There must be two portions of matter exerting forces on each other in order that the ether 
should be thrown into agitation. In ordinary line spectra we consider that the two portions of matter 
form part of the same molecule. Now, it seems possible that also two portions of different molecules 
should in their rapid approach towards each other, or recession from each other, cause forces in the ether 
which produce vibration. These latter vibrations we might expect not to take place in fixed periods, but 
to produce what we call a continuous specti’um. We may suppose that at the lowest temperature at 
which, for instance, oxygen is luminous, the vibrations in the ether are chiefly produced by this rapid 
relative motion of different molecules, while at higher temperatures the relative motions of different 
portions of one molecule might have the upper hand; the continuous spectrum in one case, and the line 
spectrum in the other, might thus be explained. 
I admit that there is much to be said in favour of this view, and must leave to further experimentation 
to decide whether it is the one most accordant with facts or not. That the discontinuous spectra of 
different orders (line and band spectra) are due to different molecular combination, I consider to be 
pretty well established, and analogy has led me (and Mr. Lockyeb before me) to explain the continuous 
spectra by the same cause ; for the change of the continuous spectrum to the line or hand spectrum takes 
place in exactly the same way as the change of spectra of different orders into each other. Analogy is 
not a strong guide, yet some weight may be given to it in a case like the one under discussion, where 
experiment hitherto has failed to give a decided answer. Professor Stokes has also suggested that the 
continuous spectrum which comes out under high pressures may really be only the continuous spectrum 
of low temperatures. At such pressures the discharge of an induction coil consists of a bright and 
sensibly instantaneous spark, followed by an apparently continuous discharge, for which the spark 
appears to open a path ; and it is possible that the line spectrum may really belong to the spark, and the 
continuous background to the continuous discharge. Professor Stokes has suggested an experiment 
with a rotating mirror to decide the point: an experiment which I hope to be able to make before 
long.—Note added December 1, 1878. 
