9() CELESTIAL SPECTROSCOPY. 
begun. It is worthy of remark that these nebule group themselves 
about the Milky Way, where we find a preponderance of the white-star 
type of stars, and almost exclusively the bright-line stars which Pick- 
ering associates with the planetary nebule. Further, Dr. Gill con- 
cludes, from the rapidity with which they impress themselves upon the 
plate, that the fainter stars of the Milky Way also, to a large extent, 
belong to this early type of stars. At the same time other types of 
stars occur also over this region, and the red hydrocarbon stars are 
found in certain parts; but possibly these stars may be before or behind 
the Milky Way, and not physically connected with it. 
If light matter be suggested by the spectrum of these nebul, it 
may be asked further, as a pure specwation, whether in them we are 
witnessing possibly a later condensation of the light matter which had 
been left behind, at least in a relatively greater proportion, after the 
first growth of worlds into which the heavier matter condensed, though 
not without some entanglement of the lighter substances. The wide 
extent and great diffuseness of this bright-line nebulosity over a large 
part of the constellation of Orion may be regarded perhaps as _ point- 
ing in this direction. The diffuse nebulous matter streaming round the 
Pleiades may possibly be another instance, though the character of its 
spectrum has not yet been ascertained. 
In the planetary nebulie, as a rule, there is a sensible increase of the 
faint continuous spectrum, as well as a slight thickening of the bright 
lines toward the center of the nebula, appearances which are in favor 
of the view that these bodies are condensing gaseous masses. 
Prof. G. Darwin, in his investigation of the equilibrium of a rotating 
mass of fluid, found, in accordance with the independent researches of 
Poincaré, that when a portion of the central body becomes detached 
through increasing angular velocity, the portion should bear a far larger 
ratio to the remainder than is observed in the planets and satellites of 
the solar system, even taking into account heterogeneity from the con- 
densation of the parent mass. 
Now this state of things, in which the masses though not equal are 
of the same order, does seem to prevail in many nebule, and to have 
given birth to a large class of binary stars. Mr. See has recently in- 
vestigated the evolution of bodies of this class, and points out their 
radical differences from the solar system in the relatively large mass- 
ratios of the component bodies, as well as in the high eccentricities of 
their orbits, brought about by tidal friction, which would play a more 
important part in the evolution of such systems. 
Considering the large number of these bodies, he suggests that the 
solar system should perhaps no longer be regarded as representing 
celestial evolution in its normal form— 
A goodly Paterne to whose perfect mould 
He fashioned them - - - — 
but rather as modified by conditions which are exceptional. 
