348 
POPULAR SCIENCE REVIEW. 
one consisting of bright lines only. Fig. 15 is copied from a 
drawing by Lord Eosse of a nebula, afterwards examined by 
!Mr. Huggins, and a representation of the spectrum which he 
observed, and which proves that this particular nebula consists 
of glowing gas without any central solid or liquid nucleus. 
About twenty out of sixty 
nebulae examined by Mr. Hug- 
gins formed spectra composed 
of bright lines only. Of the 
rest, most give a faint con- 
tinuous spectrum, as though 
these were really in a more 
advanced stage of condensa- 
tion than the gaseous nebulae. 
In all these spectra a bright 
line coincident with one of 
the ’ bright lines of nitrogen 
occurs, so that they appear all 
to have a common character, 
and contain the same elementary substance. In a few of the 
brighter nebulae, three or even four lines have been observed, 
as in the instance figured above ; but the position of each of 
tliese lines is in all cases the same, when compared with the 
spectra of other nebulae. The position of the third line coin- 
cides with that of the most prominent line in the spectrum of 
hydrogen, so that there can be little doubt that the elementary 
gases, hydrogen and nitrogen, in a state of high ignition, are 
the chief components of these remarkable bodies. 
And now let us endeavour to form some notion of the dis- 
tances of these bodies, of which the constitution and chemical 
nature have thus in part been made known to us. 
The diameter of the earth on which we live is nearly 8,000 
miles, and the moon is at about thirty times this distance from 
us, while the sun is 380 times as hir off as the moon. How can 
we in any way picture to ourselves these immense distances? 
Suppose that the sun were represented by a globe 2 ft. in dia- 
meter, the earth would then be of the size of a pea, and it would 
be placed at a distance of 215 ft. from it, or about twice as 
far off as I am from the wall of this room in front of me ; and 
the moon would be of the size of a mustard seed placed 7 in. 
from the pea, which represents the earth ; whilst Neptune, the 
most distant of the planets, would be of the size of a large plum, 
and would l)C placed at a mile and a quarter from the 2 ft. globe 
supposed to represent the sun. 
Well, Sirius, the brightest of the fixed stars, if measured by 
this scale, would be 40,000 miles away from us, or at a distance 
five times as great as that which now separates us in a straight 
Fig. 15. 
