e excursions of opinion on the
point were abruptly restricted and defined by the application to them of
the spectroscope. On August 29, 1864, Sir William Huggins sifted through
his prisms the rays of a bright planetary nebula in Draco.[1510] To his
infinite surprise, they proved to be mainly of one colour. In other
words, they avowed their origin from a mass of glowing vapour. As to
what _kind_ of vapour it might be by which Herschel's conjecture of a
"shining fluid" diffused at large throughout the cosmos was thus
unexpectedly verified, an answer only partially satisfactory could be
afforded. The conspicuous bright line of the Draco nebula seemed to
agree in position with one emitted by nitrogen, but has since proved to
be distinct from it; of its two fainter companions, one was unmistakably
the F line of hydrogen, while the other, in position intermediate
between the two, still remains unidentified.

By 1868 Huggins had satisfactorily examined the spectra of about seventy
nebulae, of which one-third displayed a gaseous character.[1511] All of
these gave the green ray fundamental to the nebular spectrum, and
emanating from an unknown form of matter named by Sir William Huggins
"nebulum." It is associated with seven or eight hydrogen lines, with
three of "yellow" helium, and with a good many of undetermined origin.
The absence of the crimson radiation of hydrogen--perceived with
difficulty only in some highly condensed objects--is an anomaly very
imperfectly explained as a physiological effect connected with the
extreme faintness of nebular light.[1512] An approximate coincidence
between the chief nebular line and a "fluting" of magnesium having been
alleged by Lockyer in support of his meteoritic hypothesis of nebular
constitution, it became of interest to ascertain its reality. The task
was accomplished by Sir William and Lady Huggins in 1889 and 1890,[1513]
and by Professor Keeler, with the advantages of the Mount Hamilton
apparatus and atmosphere, in 1890-91.[1514] The upshot was to show a
slight but sure discrepancy as to place, and a marked diversity as to
character, between the two qualities of light. The nebular ray
(wave-length 5,007 millionths of a millimetre) is slightly more
refrangible than the magnesium fluting-edge, and it is sharp and fine,
with no trace of the unilateral haze necessarily clinging even to the
last "remnant" of a banded formation.

Planetary and annular nebulae are, without exception, ga