Dec. 16, 1869] 
NATURE 
185 
to the sun. Part of its light is, no doubt, merely sunlight 
reflected from the matter of the sun’s atmosphere, or from 
cosmical bodies revolving about the sun; for it has long 
been known to be partially polarised. But it is only 
within the last few months that zfs spectrum also has 
been observed, and found to consist mainly of bright 
lines—z.e. of a few rays of definite refrangibility. The 
positions of the most marked of these have been mea- 
sured, and they are found to correspond with those of the 
light of terrestrial auroras! This is one of the most 
startling results yet obtained by observation; for the 
aurora is intimately connected with, or at all events has 
an important effect on, terrestrial magnetism, and it has 
been known for some time that disturbances in the sun 
have a marked effect on the magnetism of the earth. 
Our sun is a variable star. It has been proved that its 
spots have an eleven-year period of maximum frequency. 
Laborious calculations are now in progress at Kew Obser- 
vatory, with the view of tracing the cause of this periodic 
effect; and it seems already to be traced with some 
certainty to the planets, principally to Mercury, Venus, 
and Jupiter; the first, though very small, being very 
near, and the last, though very distant, being very large. 
Now, the red flames, or hydrogen clouds, are intimately 
associated with sun-spots. Hence we connect ¢hezr fre- 
quency with the variability of sunlight. Now, it is only a 
year or two since an exceedingly well-marked case of a 
temporary star was visible in the northern hemisphere ; a 
star, usually of inconsiderable magnitude, scarcely visible 
to the naked eye, suddenly blazed out with brightness 
rivalling that of Sirius. The spectroscope showed that it 
owed this increase of its light almost solely to incandescent 
hydrogen, the chief material of the flame-cloud that hovers 
over a solar spot. 
Nor is it only in solar and stellar phenomena that these 
extraordinary recent advances have been made. Bodies 
even more puzzling and anomalous than the sun and stars 
are common enough in the universe. Many nebule, long 
imagined to be immense groups of stars, at such enormous 
distances that the several constituents were indistinguish- 
able by the most powerful telescopes, have been shown to 
shine as glowing gas merely, rendering it probable that 
we have to deal with objects which, though certainly at 
vast distances from the earth, are probably not vastly 
farther away than some of the nearest stars. Possibly, in 
some cases, they may be much nearer, in which case they 
may be suns which have cooled, and are still surrounded 
by glowing gas, due to the impacts of small cosmical 
masses, or meteorites, on or near their surface. Or they 
may be vast systems of small cosmical masses in the act 
of grouping themselves by mutual gravitation, impact, and 
friction into a new star, the incandescent gas being due 
to the impacts and the friction. In them we may be 
actually watching the formation of a solar system. 
Finally, let us consider what we have recently learned 
about comets—bodies which have hitherto puzzled the 
astronomer quite as much as have the nebula. Several 
ingenious speculations have recently been published on 
this very interesting subject, but I shall only mention one 
with any detail. There seem to be good grounds for 
imagining that a comet is a mere shower of stones 
(meteorites and fragments of iron). This at least is cer- 
tain, that such a shower would behave, in its revolution 
about the sun, very much as comets are seen to do, and 
that, as we have reason to believe is the case with comets, 
it would be drawn out after a few revolutions, if it de- 
scribed a closed path, so as to be spread over the greater 
part of its orbit. If the earth, then, were at any time to 
intersect the orbit of the comet, it would pass through a 
stream of such stones, all moving approximately in parallel 
lines and with equal velocities. On entering the earth’s 
atmosphere with the enormous relative velocity due to 
revolution aboutthe sun in differently sized orbits, described 
sometimes with a retrograde motion, these fragments 
of stone would, by the laws of perspective, describe paths 
all apparently diverging from one point in the heavens, 
and these paths would be rendered visible by the incan- 
descence of the meteorites due to friction of the air. Now 
this is exactly what we see, markedly in August and 
November every year, less definitely at other fixed periods. 
And the orbits of the August and November meteorites 
have been determined, and found to be identical with 
those of two known comets. I cannot enter very fully 
into this most interesting subject now, but I may say 
a few words more in explanation. Unfortunately, since 
spectroscopes have been in everybody’s hand, no notable 
comets have appeared. [How strange it wow seems to us 
that the magnificent comets of 1858 and 1860 were allowed 
to pass without having been looked at through a prism by 
anyone, whether as a matter of chance or of curiosity!] 
Such small comets as have been observed have given 
continuous spectra from their tails, so far as could be 
judged with regard to an object so feebly illuminated. 
This, then, it would appear, is simply reflected solar light. 
The heads, however, give spectra somewhat resembling 
those of the nebula I have just mentioned—the spectra 
of incandescent gases. This is quite consistent with the 
descriptions given by Hevelius and others of some of the 
grander comets ; which presented no peculiarities of colour 
in the tail, but where the head was blueish or greenish. Now 
these appearances are easily reconciled with the shower-of 
stones hypothesis. For the nucleus, or head, of a comet 
is that portion of the shower where the stones are most 
numerous, where their relative velocities are greatest, 
and where, therefore, mutual impacts, giving off incan- 
descent gases, are the most frequent and the most violent. 
This simple hypothesis explains easily many very striking 
facts about comets, such as their sometimes appearing to 
send off 7 a few hours a tail many hundreds of millions 
of miles in length. Wild notions of repulsive forces 
vastly more powerful than the sun’s gravity have been 
entertained ; bold speculations as to decomposition (by 
solar light) of gaseous matter left behind it in space by 
the comet have also been propounded ; but it would seem 
that the shower-of-stones hypothesis accounts very simply 
for such an appearance. For, just as a distant flock of 
seabirds comes suddenly into view as a dark line when 
the eye is brought by their evolutions into the plane in 
which they fly, so the scattered masses which have lost 
velocity by impact, while they formed part of the head, or 
those which have been quickened by the same action, 
as well as those which lag behind the others in virtue 
of the somewhat larger orbits which they describe, show 
themselves by reflected solar light as a long bright streak 
whenever the earth moves into any tangent plane to the 
surface in which they are for the time mainly gathered. 
