The next considerable revelation was obtained from 
the photograph, taken in 1889, of the spiral nebula—long 
recognised as such—in Canes Venatici, certainly one 
of the most wonderful spiral nebulz in the heavens. It is 
all the more striking because this is a nebula which we 
NEBULA ANDROMEDA 
INTEGRATION — - 
i tos 
NOVA ANDROMEDA ocr. 
Ta 2" 
COMETI 1868 
Fic. 2.—Spectrum of the nebula in Andromeda compared with Nova Andromedz and comet. 
look down upon ; we see it in plan ; we are, so to speak, | 
at the pole of the system, so that it is not foreshortened. | 
There is no question about the wonderful spirals being | 
connected with the central condensation and stretching | 
Fic. 3.—The spiral nebula in Canes. Venatici, from a photograph by 
Dr. Roberts, x 
towards it. I call attention to the points of conden- 
‘sation along one of the spiral branches, and where 
we get the possible intrusion of two spirals one on the 
other we see a confused mass of light. Now, if we 
NO. 1420, VOL. 55] 
NATORE. 
| JANUARY 14, 
1897 
imagine ourselves dealing there with a mass of pure gas, 
whether it is hydrogen or nitrogen or ammonia—that is, 
a combination of both—or any other, it would be ex- 
tremely difficult to see why there should be any change 
of temperature in different parts of that mass ; but the 
The flutings common to all are those of carbon. 
moment we assume that we are dealing with cool 
materials—meteoritic dust—we see that such a picture as 
this is important, for the reason not that it shows us what 
is there, but because it shows us what is going on there, 
as already pointed out in relation to the nebula of Andro- 
meda. The bright spots do not represent the presence 
of matter merely, and the dark ones its absence; but 
the brighter portions represent the intersection of stream 
lines where collision is possible—the intervals those 
regions where collisions are less likely. We can gather 
from the very configuration of this system that.if all the 
dust, or meteorites, or conglomerations of particles, what- 
ever they may be, are going the same way, there will be 
a condition in which we shall get a minimum of collisions, 
and thérefore a minimum of temperature. If the move- 
ments are quite orderly and in the same direction, we 
must not expect to get any very great disturbance, and 
therefore—if these disturbances produce high tempera- 
tures—we shall not expect to get indications of any 
particularly high temperatures. 
The important point is that here we get apparent stars 
arranged along the spirals. 
Dr. Roberts writes as follows :— 
“The photograph shows both nuclei of the nebula to 
be stellar, surrounded by dense nebulosity, and the con- 
volutions of the spirals. in this as in other spiral nebule 
are broken up into star-like condensations with nebu- 
losity around them. Those stars which do not conform 
to the trends of the spirals, have nebulous trails attached 
to them.” + 
Strikingly similar to the above is the photograph of — 
M 74 Piscium taken by Dr. Roberts in December 1893 ; 
and here again it is also a question of apparent stars. 
“The photograph shows the nebula to be a very per- 
fect spiral, with a central stellar nucleus and a 15 mag. 
star close to it on the south side. The convolutions of 
the spiral are studded with many stars and star-like con- 
densations, and on the north preceding side there isa 
partial inversion of one of the convolutions, which con- 
veys the idea of some irregular disturbing cause having 
interfered with the regular formation of a part of that 
convolution.” * 
In Messier 101 Urs Majoris, which was photographed 
in May 1892, we have another case in which the con- 
volutions are broken up into star-like condensations.’ 
1 Roberts’ photographs, p. 85. 2 M_N., vol liv. p. 438 
3 Roberts’ photographs, p. 89. 
