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XXVII. On the Long Spectrum of Electric Light. By G. G. Stokes, M.A.., D.C.L., 
Sec. B.S., Lucasian Professor of Mathematics in the University of Cambridge. 
Eeceived June 19, — Eead June 19, 1862. 
Introduction. 
The experimental researches described in a former paper * led me indirectly to the con- 
clusion that the electric spark, whether obtained directly from the prime conductor of 
an ordinary electrifying machine, or from the discharge of a Leyden jar, emits rays of 
very high refrangibility, surpassing in this respect any that reach us from the sun — and 
that these rays pass freely through quartz, while glass absorbs them, as it does also the 
most refrangible of the solar rays. I was induced in consequence to procure prisms and 
a lens of quartz, which were applied in the first instance to the examination of the solar 
spectrum, and which immediately revealed the existence of an invisible region extending 
as far beyond that previously known as the latter extends beyond the visible spectrum, 
and exhibiting a continuation of Feaunhofee’s lines f. A map of the new lines was 
exhibited at an evening lecture delivered before the British Association at their Meeting 
in Belfast in the autumn of the same year ; and I then stated that I conceived we had 
obtained evidence that the limit of the solar spectrum in the more refrangible direction 
had been reached. In fact, the very same arrangement which revealed, by means of 
fluorescence, the existence of what were evidently rays of higher refrangibility coming 
from the electric spark failed to show anything of the kind when applied to the solar 
spectrum. At least, the only link in the chain of evidence which remained to be sup- 
plied by direct experiment related to the reflecting power, for rays of high refrangibility, 
of the metallic speculum of the heliostat which was employed to reflect the sun’s rays 
into a convenient direction ; and this was shortly afterwards tested by direct experiment, 
on rays from an electric discharge separated by prismatic refraction. 
In making preparations for a lecture on the subject delivered at the Royal Institution 
in February 1853, in which I had the benefit of the kind assistance of Mr. Faeadat, 
recourse was naturally had to electric light, on account of the extraordinary richness 
which it had been found to possess in rays of high refrangibility. Although fully pre- 
pared to expect rays of much higher refrangibility than were found in the solar spectrum, 
I was perfectly astonished, on subjecting a powerful discharge from a Leyden jar to 
prismatic analysis with quartz apparatus, to find a spectrum extending no less than six 
or eight times the length of the visible spectrum, and could not help at first suspecting 
that it was a mistake arising from the reflexion of stray light. A similarly extensive 
* “ On the Change of Eefrangibility of Light,” Phil. Trans, for 1852, p. 463. f Ibid. p. 559. 
MDCCCLXII. 4 N 
