ed it to him as a mere
shimmer at the last limit of visibility; and it came out in three
diffuse patches on plates to which, on December 6 and 27, 1899, Keeler
gave prolonged exposures with the Crossley reflector.[1521] Moreover, a
fairly bright adjacent nebula, perceived by O. Struve in 1868, and
observed shortly afterwards by d'Arrest, has totally vanished, and was
most likely only a temporary apparition. These are the most authentic
instances of nebular variability. Many others have been more or less
plausibly alleged;[1522] but Professor Holden's persuasion, acquired
from an exhaustive study of the records since 1758,[1523] that the
various parts of the Orion nebula fluctuate continually in relative
lustre, has not been ratified by photographic evidence.

The case of the "trifid" nebula in Sagittarius, investigated by Holden
in 1877,[1524] is less easily disposed of. What is certain is that a
remarkable triple star, centrally situated, according to the
observations of both the Herschels, 1784-1833, in a dark space between
the three great _lobes_ of the nebula, is now, and has been since 1839,
densely involved in one of them; and since the hypothesis of relative
motion is on many grounds inadmissible, the change that has apparently
taken place must be in the distribution of light. One no less
conspicuous was adduced by Mr. H. C. Russell, director of the Sydney
Observatory.[1525] A particularly bright part of the great Argo nebula,
as drawn by Sir John Herschel, has, it would seem, almost totally
disappeared. He noticed its absence in 1871, using a 7-inch telescope,
failed equally later on to find it with an 11-1/2-inch, and his
long-exposure photographs show no vestige of it. The same structure is
missing from, or scarcely traceable in, a splendid picture of the nebula
taken by Sir David Gill in twelve hours distributed over four nights in
March, 1892.[1526] An immense gaseous expanse has, it would seem, sunk
out of sight. Materially it is no doubt there; but the radiance has left
it.

Nebulae have no ascertained proper motions. No genuine change of place in
the heavens has yet been recorded for any one of them. All equally hold
aloof, so far as telescopic observation shows, from the busy journeyings
of the stars. This seeming immobility is partly an effect of vast
distance. Nebular parallax has, up to the present, proved evanescent,
and nebular parallactic drift, in response to the sun's advance through
space, rem