SCIENTIFIC SUMMARY. 
ASTRONOMY. 
are the Nebulae ? — A paper by Mr. Stone has recently been 
communicated to the Royal Society, in which he actually bases on 
Huggins’s discovery of the bright line spectrum of certain nebulae the theory 
that these are clusters of stars and not gaseous masses. His reasoning is 
remarkable. “ The sun is known to be surrounded by a gaseous envelope 
of very considerable extent. Similar envelopes must surround the stars 
generally. Conceive a close stellar cluster. Each star, if isolated, would 
be surrounded by its own gaseous envelope. These gaseous envelopes 
might, in the case of a cluster, form over the whole, or a part of the cluster,, 
a continuous mass of gas. So long as such a cluster was within a certain 
distance from us the light from the stellar masses would predominate over 
that of the gaseous envelopes. The spectrum would therefore be an ordinary 
stellar spectrum. Suppose such a cluster to be removed further and further 
from us ; the light from each star would be diminished in the proportion of 
the inverse square of the distance ; but such would not be the case with, 
the light from the enveloping surface formed by the gaseous envelopes. The 
light from this envelope, received on a slit in the focus of an object-glass, 
would be sensibly constant, because the contributing area would be increased 
in the same proportion that the light from each part is diminished. The 
result would be that at some definite distance, and all greater distances, the 
preponderating light received from such a cluster would be derived from 
the gaseous envelopes and not from the isolated stellar masses. The 
spectrum of the cluster would therefore become a linear one, like that 
from the gaseous surroundings of our own sun.” It seems hardly credible that 
a former first assistant at Greenwich and a mathematician of Mr. Stone’s 
acknowledged power should reason in this wise. As shown in 1870, by 
Mr. Proctor, in a paper on resolvability as a test of distance (“ Monthly 
Notices of the Roy. Astr. Soc.” vol, xxx.), an irresolvable star-cluster would 
retain its intrinsic brightness unchanged, however its distance varied, so 
long as it continued irresolvable. For as the stars (separately undiscern- 
ible) became fainter with distance, the area over which they would be 
scattered would become smaller, and in the same degree. Dr. Huggins 
has thought it necessary to reply to Mr. Stone’s paper! “Waiving the 
objections which may be urged against Mr. Stone’s reasoning,” he considers 
the results of observation. He points out (1) that there are not found in 
the spectra of different nebulae the differences of relative brightness of the 
