Nov. 25, 1869] 
NATURE 
115 
ASTRONOMY 
Kirkwood on the Origin of Comets 
THE recent important investigations of Hoek on the origin of 
comets may be said to have opened out quite a new field of 
astronomical research of the highest importance. We are glad, 
therefore, to lay before our readers an abstract of a continuation 
of the work which we owe to Professor Kirkwood, who has 
communicated it to Si/imans’ Journal. Professor Kirkwood 
has dealt with the comets 1812, i. and 1846, iv. 
The wonderful similarity of the elements of these, except in 
the longitude of the ascending node, is very remarkable. It is 
also noticeable that the longitude of the descending node of the 
latter is very nearly coincident with that of the ascending node of 
the former. These remarkable coincidences are presented to the 
eye in the following diagram, where the dotted ellipse represents 
the orbit of the comet of 1812, and the continuous curve, that of 
the comet of 1846. 
/, N 
NEPTUNE _ 
SN ee 
\\ = 
— 
\ ft \\ 
Vir r| 
\ |! _unanus z | 3 
SN09 
_€/8/ 30 L309 
7 50 L 
"ebE 
Dr. Kirkwood remarks :— 
“Tt is infinitely improbable that these coincidences should be 
accidental : they point, undoubtedly, to a common origin of the 
two bodies.” And adds :— 
“The theory of comets now generally accepted is that they enter 
the solar system aé extra, move in parabolas or hyperbolas around 
the sun, and, if undisturbed by the planets, pass off beyond the 
limits of our system to be seen no more. If in their motion, 
however, they approach very near any of the larger planets, their 
direction. is changed by planetary perturbations; their orbits 
being sometimes transformed into ellipses. The new orbits of 
such bodies would pass very nearly through the points at which 
their greatest perturbation occurred: and accordingly we find 
that the aphelia of a large proportion of the periodic comets are 
near the orbits of the major planets, ‘I admit,’ says M. Hoek, 
‘that the orbits of comets are by nature parabolas or hyperbolas, 
and that in the cases when elliptical orbits are met with, these 
are occasioned by planetary attractions, or derive their character 
from the uncertainty of our observations. To allow the contrary 
would be to admit some comets as permanent members of our 
planetary system, to which they ought to have belonged since its 
origin, and so to assert the simultaneous birth of that system and 
of these comets. As for me, I attribute to these a primitive 
wandering character. Travelling through space they move from 
one star to another in order to leave it again, provided they do 
not meet any obstacle that may force them to remain in its 
vicinity. Such an obstacle was Yfzter, in the neighbourhood of 
our sun, for the comets of Lexell and Brorsen, and probably for 
the greater part of periodical comets; the other part of which 
may be indebted for their elliptical orbits to the attractions of 
Saturn and the remaining planets. 
*** Generally, then, comets come to us from some star or other. 
The attraction of our sun modifies their orbit, as had been done 
already by each star through whose sphere of attraction they had 
passed, We can put the question if they come as single bodies 
or united in systems. This is the point I have undertaken to 
investigate. Since some time already I had felt the truth of the 
following thesis :— 
“* There are systems of comets in space that are broken up by the 
attraction of our sun, and whose members attain, as isolated bodies, 
the vicinity of the Earth during a course of several years.’ 
“In the researches here referred to ithas been shown by M. 
Hoek that the comets of 1860 iii, 1863 i., and 1863 vi., 
formed a group in space previous to their entrance into our 
system. The same fact has also been demonstrated in regard to 
other comets which need not here be specified. Now, the comets 
of 1812 and 1846, iy. have their aphelions very near the orbit of 
Neptune, and hence the original parabolas in which they moved 
were probably transformed into ellipses by the perturbations of 
that planet. Before entering the solar domain they were doubt- 
less members of a cometary system, Passing Neptune near the 
same time, and at some distance from each other, their different 
relative positions with regard to the disturbing body may account 
for the slight differences in the elements of their orbits. 
‘© At what epoch did they enter the solar systen ? The mean be- 
tween the longitudes of the aphelia of the two comets is 271° 41’. 
Neptune had this longitude in 1775 ; the comet of 1812, in 17773 
and that of 1846, in 1809. Now, with the known period of 
Neptune and the periods of the comets as determined by Encke 
and Peirce, we find (neglecting perturbations) that— 
Neptune was in longitude 271° 41’ in the year 694 B.C. 
The Cometof 1812 ,, - Ar 696 ,, 
” 1846, iv. 2) ” ” 696 ” 
It seems, therefore, that the three bodies were very nearly to- 
gether about 69§ years before the Christian era. It is conse- 
quently not improbable that the elliptical form of the two 
cometary orbits dates from this epoch.” 
BOTANY 
Spectroscopic Examination of Diatoms 
THE vegetable nature of the Diatomacez is now generally ad- 
mitted, but if any further proof were needed we have it in marked 
results from the application of the spectroscope. Mr. H. L. Smith 
has been enabled to prove the absolute identity of chlorophyll, or 
the green endochrome of plants, with diatom, or the olive yellow 
endochrome of the Diatomaceze. The spectrum-microscope used 
was made by Browning, of London. Mr. Smith states that it 
is not at all difficult to obtain a characteristic spectrum from a 
living diatom, and to compare it directly with that of a desmid, 
or other plant. From about fifty comparisons of spectra, he 
concludes that the spectrum of chlorophyll is identical with that 
of diatomin. The spectrum in question is a characteristic one, 
and is figured below, 
A very black, narrowish band in the extreme red, reading at the 
lower edge, which appears to be constant, about } of Mr. Sorby’s 
scale, is too characteristic to be mistaken. ‘There are two other 
very faint bands, not easily seen, and somewhat more variable in 
position. The black band in the red is always present, and is 
remarkably constant in the position of its lower edge. In making 
comparisons of spectra it is of the utmost importance that the 
slit of the spectroscope should be absolutely in the focus of the 
achromatic eye lens. If this be not attended to there willbe a 
slight parallax ; and bands really identical in position, e.g., those 
of blood (scarlet cruorine), will not absolutely correspond when 
