January 14, 1897 | 
CGELESITAL EDDIES: 
PROPOSE in the present paper to discuss the ques- 
tion whether the long-exposure photographs of Dr. 
Roberts and others, justify or negative the view, founded 
upon a large mass of spectroscopic evidence, which I put 
forward in 1887, before any of them had been published. 
The view in question was thus stated :1 “ The brighter 
lines in spiral nebulze, and in those in which a rotation 
has been set up, are in all probability due to streams of 
meteorites, with irregular motions out of the main streams 
in which the collisions would be almost 727.” 
I was careful to state that Prof. G. Darwin, when dis- 
cussing the gaseous hypothesis of Laplace, had already 
pointed out that “the great mass of the gas is non- 
luminous, the luminosity being an evidence of condensa- 
tion along lines of low velocity, according to a well-known 
hydrodynamical law. From this point of view the 
small nebula may be regarded as a luminous diagram 
of its own stream-lines.”? 
At the time I wrote in 1887, the nebula in Andromeda 
was not considered to be a spiral nebula. The most 
striking representation of it was due to Bond, who drew 
special attention to two black streaks running nearly 
parallel to the longer diameter. 
It may also be added that in 1887 we knew nothing for 
certain about its spectrum. 
In 1888, Dr. Isaac Roberts published his most admir- 
able long-exposure photographs, which at once established 
the spiral nature of the nebula ; and in the same year 
the complete discussion of the spectroscopic observations 
made up to that time led me to predict that if the nebulze 
were carefully observed we should find in them, sooner or 
later, indications of the substance which makes the comet 
spectrum so very distinct and special. In 1889, that is 
in the next year, the spectrum of carbon was discovered 
by Mr. Fowler and Mr. Taylor in the nebula of Andro- 
meda 
I will take the photograph first. The plane of move- 
ment in the spiral system is so situated that from our 
point of space we look at it obliquely ; hence the nebula 
appears very elliptic. Still there is no difficulty in seeing 
that the various streams round the centre of conden- 
sation are all of them of a spiral form, with certain 
condensations interspersed here and there along them. 
We have a condensation in the prolongation of one of 
the spirals, and there is considerable clustering of apparent 
stars along the stream lines. It is important to indicate 
that we have in these appearances, not signs which tell us 
of the existence of matter merely—so that when we have 
not the appearances we would be justified in supposing that 
there was no matter—but an indication of movement in 
matter, so that we may imagine that this nebula and 
others like it do probably consist of something extending 
enormously in space beyond the indications w hich we see, 
for the reason that near the centre the movements are 
more violent than they are towards the outside. We are 
there face to face with the idea that we have to deal with 
orderly movements. If the movements are orderly, it 
means that the movements of the constituent particles of 
the swarm, all of them, or most of them, will be in the 
same direction ; in that case we have the condition of 
minimum disturbance, and therefore the condition of 
minimum temperature. 
In short, not only have we regular spirals, but in addi- 
tion to the spiral system there seems to be revealed 
irregular masses of nebula near both ends of the major 
axis. Where more than one stream seems to be con- 
tending, the brilliancy is enhanced, and much irregular 
luminosity is apparent. On the other hand, in the part 
of the main nebula most free from these irregularities 
the spirals are almost invisible. 
1 P.R.S., November 17, 1887, p. 153. 
2 NaTuRE, vol. xxxi. p. 25- 
NO. 1420, VOL. 55] 
VE CLE 249 
Next for the spectroscopic observations. 
The chief argument urged in favour of the gaseous 
nature of the nebulz now is the existence of hydrogen 
and helium in the planetary nebulz and in such a nebula 
as that of Orion ; the unknown form of nitrogen has no 
longer any votaries. 
But if the spiral nebulee be gaseous, why do they not 
give us the spectra of hydrogen and helium? The 
spectrum of the nebula of Andromeda is practically the 
spectrum of a comet, and therefore we are justified in 
considering it as built up of cometary materials. Now 
these, as is generally conceded, are meteoritic in their 
nature. 
But this is not all the evidence bearing upon this 
question, even so far as regards the nebula we are now 
discussing. Not many years agoa new star was observed 
Fic. 1.—Nebula of Andromeda, 1887. 
and the difference between the spectra of 
the new star and of the nebula itself was merely the 
addition of the lines of hydrogen! On the meteoritic 
hypothesis this is easily explained by an increased number 
of collisions lasting fora time : on the gaseous hypothesis 
an explanation is not so easy. 
If it be granted that we are really dealing with 
streams of meteorites, all the new phenomena revealed 
to us by Dr. Roberts’ photographs receive a simple and 
sufficient explanation, especially the apparent condensa- 
tions here and there, which are not condensations of 
matter necessarily but /oc’ of greater disturbances caused 
by crossing streams. 
in the nebula, 
