2 
ASTRONOMY: G. F. BECKER 
matter spread out in diverse bodies in different parts of the heavens of which 
it occupies a great area. He saw in some of these bodies the nebulous matter 
slightly condensed about one or several sHghtly luminous nuclei. In other 
nebulae, these nuclei shone more brightly relatively to the nebulosity which 
surrounded them. The atmosphere of each nucleus was about to segregate 
by a final condensation: multiple nebulae resulted formed of brilliant nuclei 
very close together and each surrounded by an atmosphere: occasionally the 
nebulous matter condensing uniformly produced the nebulae called planetary. 
Finally a greater degree of condensation transformed all of these nebulae 
into stars. The nebulae classified according to this philosophic view indicate 
with extreme probability their future transformation into stars and the former 
nebulous state of the stars now in existence. 
It will be observed that nebulae with a single nucleus are mentioned 
only as special cases of multinuclear and, therefore, heterogeneous neb- 
ulae. Herschel himself made repeated references to the character of 
nebulous matter. For him it was anything which could shine, and 
while he was unable to come to definite conclusions, he regarded it as 
probable that it comprised matter in all three states. 
Laplace's information as to nebulae was derived almost wholly from 
Herschel; so far as I can ascertain he made no telescopic observations 
of his own on these bodies, and his ideas of the constitution of nebulae 
seem to have been identical with those of the great English observer. 
Both in the passage quoted above and in his famous note on the nebular 
hypothesis Laplace referred to atmospheres, but they were cloudy or 
dusty atmospheres not inconsistent with the presence of multiple nuclei; 
they were characterized by a 'numberless variety of densities' and seem 
to have resembled volcanic clouds. In other essays, such as that on 
the barometer, Laplace showed his complete mastery of Boyle's and 
Gay Lussac's laws and in these memoirs he frequently employs the word 
gas. In his note on the nebular hypothesis he never once uses this word 
and the very first application he made of his theory was to offer an 
explanation of the genesis of the Pleiades. In his time, the presence 
of nebulous matter in this cluster was unknown, but the photographic 
plate shows that it is embedded in one of the most astonishing of the 
great nebular masses. Laplace's selection of the Pleiades was thus a 
striking example of his prescience. 
Much as more modern science has contributed to knowledge of the 
nebulae, the question of their constitution is still unsettled. That 
some of them contain luminous gas (helium, hydrogen, nebulium) was 
shown by Huggins 50 years ago, but many nebulae and among them 
the spirals show continuous spectra. The continuity might be due to 
