SCIiiNTIFIC SUMMARY. 
309 
succeeded in solving tlie difficult problem we have indicated above. He 
has made use of a spectroscope having a dispersive power seven times as 
great as that of a single equiangular prism of crown glass. This powerful 
instrument failed to indicate any signs of motion in the great Orion nebula 
and some others of the gaseous nebulae which Mr. Huggins examined by 
its means. But he was more successful with the bright star Sirius. Having 
satisfied himself that a well-marked line in the spectrum of this star really 
corresponds with the bright line f of hydrogen, he brought the spectra of 
Sirius and of incandescent hydrogen into direct comparison. He found that 
the line f of Sirius was separated by about the 250th part of an inch from 
the corresponding line in the spectrum of hydrogen. The displacement was 
towards the red end of the spectrum, indicating a motion of recession between 
Sirius and the earth. When this motion has been reduced by the amount 
of motion which the earth had from Sirius at the time of observation, it 
results that Sirius has a motion of recession from the sun at the rate of no 
less than 930 millions of miles per year. Still further reducing this result, 
on account of the sun’s estimated motion towards the constellation Hercules, 
we obtain a proper motion of recession of about 780 millions of miles per 
annum. The star’s observed proper motion, which amounts to about 1^ se- 
conds annually, when combined with the best estimates of the star’s distance, 
indicates a proper transverse motion at the rate of 450 millions of miles per 
annum. Hence the actual motion of the star in sidereal space is readily shoiun 
to he about nine hundred millions of miles per annum. 
There is nothing to prevent this mode of inquiry from being applied in 
time to all the more conspicuous stars, or even to all the lucid stars, so that, 
instead of the vague notions we have been hitherto able to derive from the 
stars’ apparent proper motions, which only enable us to determine roughly 
their real transverse motions, we shall be enabled to judge of the amount 
and direction of their actual motions through sidereal space. 
BrorserHs Comet . — The re-discovery of Brorsen’s comet has been hailed with 
more interest than would otherwise have been due to it, on account of the fact 
that Biela’s comet was not seen on the occasion of its last return j so that 
a certain amount of dubiety had begun to be attached to the returns of the 
members of that family of periodic comets to which Biela’s and Brorsen’s 
belong. Brorsen’s comet will now be visible for several months, as it will 
traverse the northern constellations Ursa Major and Bootes, drawing gradually 
nearer to the earth until August, after which it will recede from the earth. 
Mr. Huggins has examined this comet with the spectroscope. The spec- 
trum consists of three bright bands, on a background formed by a very faint 
continuous spectrum. In one of the bands two bright lines appear. From 
the breadth of the bands at right angles to the length of the spectrum, it is 
evident that their light comes from the coma as well as from the nucleus. 
On the other hand, the two bright lines appear to belong to the nucleus 
alone. The conclusion to be drawn from these observations would appear 
to be, that the comet shines, for the most part, by its own light, the outer 
parts only of the coma shining by reflected light. In this respect the comet 
differs from the two which IMr. Huggins had before analysed, whose comae, 
it will be remembered, shone by reflected light, the nucleus alone being 
self-luminous. The nucleus of Brorsen’s comet appears also to contain 
