i 
ut 
it is to the combined effect of these that the defined forms 
of the arms are due. Besides these two arms there are 
subsidiary arms, less well defined, and likewise trending 
towards the centre of revolution, and are constituted of 
interrupted streams of faint stars and nebulosity inter- 
mingled together; many of the stars are nebulous, and 
many are well defined: but small. The interspaces 
between the convolutions are more or less filled with 
faint nebulosity, having curves, rifts, fields, and lanes, 
without apparent nebulosity in them. They are like the 
interspaces in clouds of smoke, and cannot be classified. 
“There are outliers of nebulosity with many small well- 
defined and nebulous stars involved in them, and there 
are also isolated nebulous stars on the extreme boundaries 
of the nebula; but the evidence is strong that they are 
all related to the nebula, 
“Tt is by the study of the photographs, and not by 
descriptive matter, that we can form a true conception of 
the character of this nebula; from which we shall be 
justified, even now, in drawing some inferences as to its 
formation and further developments. To this end I may 
be permitted to suggest the following. 
AMOI 
[ JANUARY -14, 1897 
It is worth while to point out, in connection with the 
argument in favour of. the meteoritic nature of spiral 
nebulz, that there are other nebulz representing streams 
in space to which it seems almost impossible to attribute 
a purely gaseous origin. 
This branch of work is so young, that there has not 
yet been time to bring a crucial test to bear on these 
“stellar condensations” to which reference has been 
made. Jf they could be shown to be short-period variables, 
then thetr true stellar nature would be at once negatived. 
We have already in two instances obtained im- 
portant evidence on this point. In 1889, Dr. Roberts 
was good enough to allow me to enlarge a photograph of 
the nebula of Orion, on which there had been a double 
exposure. I pointed out to Dr. Roberts that the vari- 
ability in some of the stars was suggested. Although 
the exposure was a double one, some of the images were 
single and there were zavevszovs in the intensities of the 
double images. Dr. Roberts made a minute examination 
with the following results : ! 
“On examination of the dual stellar images on the 
photograph the eye immediately detects that ten of them 
have undergone considerable change in brightness or 
Fic. 7.—H 84 Come. 
“We know, with a reasonable amount of certainty, 
that both nebulous and meteoric matters exist in space ; 
and we also have some evidence that bodies in space 
have come into collision. 
“ From these premises we may infer that this nebula is 
the result of a collision of some kind; and we can 
imagine collisions of at least three kinds possible ; namely, 
(1) between two stars, (2) between two nebul, (3) be- 
tween two swarms of meteorites. 
“In the case of this nebula, which (if any) of the three 
possibilities mentioned seems to us the one most probable 
to have happened? Much might be said in favour of 
each of these suggestions, but I shall not at present 
enter into details, though I think we could readily i imagine 
that the collision of two swarms of meteorites, moving in 
opposite directions, one from the south following and the 
other from the xorth preceding, would account for the 
spiral appearance, the rotatory motion, and the smashed 
and scattered state in which the nebula is shown tojus 
upon the photographs.” 4 
1 77. N., vol. lvi. p. 70. 
NO. 1420, VOL. 55 | 
Fic, 
8.—Nebula near 52 (4) Cygni. 
magnitude during the interval of five days which elapsed 
between the two exposures. In three of the ten stars, 
the brightness has increased to the extent of from one- 
fourth to one-third the measured diameter of the stellar 
photo-image, and one star appears on the second ex- 
posure where none is shown on the first exposure. Six 
of the ten stars have diminished in brightness during 
the interval to the extent of from one-fourth to four- 
tenths, the measured diameter of the photo-image.” . . 
“JT have with due care examined the film of the 
negative under the microscope in order to see if any 
defect or evidence of defective sensibility on parts of the 
film could be traced so as to account for the variability in 
the brightness of the stellar images, but I could not find 
any such evidence, and I would of course have repeated 
the photographic experiment if the state of the sky at 
any time during the past twelve months had permitted. 
Those who possess the necessary telescopic power may 
study by eye observations the variability in these stars, 
and it is one of the functions of the photographic method 
to point out where eye observations can with advantage 
1M. N., vol. 1. p. 316. 
