ains likewise imperceptible.[1527] It may hence be presumed
that no nebulae occur within the sphere occupied by the nearer stars. But
the difficulty of accurately measuring such objects must also be taken
into account. Displacements which would be conspicuous in stars might
easily escape detection in ill-defined, hazy masses. Thus the measures
executed by d'Arrest in 1857[1528] have not yet proved effective for
their designed purpose of contributing to the future detection of proper
motions. Some determinations made by Mr. Burnham with the Lick refractor
in 1891,[1529] will ultimately afford a more critical test. He found
that nearly all planetary nebulae include a sharp stellar nucleus, the
position of which with reference to neighbouring stars could be fixed no
less precisely than if it were devoid of nebulous surroundings. Hence,
the objects located by him cannot henceforward shift, were it only to
the extent of a small fraction of a second, without the fact coming to
the knowledge of astronomers.

The spectroscope, however, here as elsewhere, can supplement the
telescope; and what it has to tell, it tells at once, without the
necessity of waiting on time to ripen results. Sir William Huggins made,
in 1874,[1530] the earliest experiments on the radial movements of
nebulae. But with only a negative upshot. None of the six objects
examined gave signs of spectral alteration, and it was estimated that
they must have done so had they been in course of recession from or
approach towards the earth by as much as twenty-five miles a second.
With far more powerful appliances, Professor Keeler renewed the attempt
at Lick in 1890-91. His success was unequivocal. Ten planetary nebulae
yielded perfectly satisfactory evidence of line-of-sight motion,[1531]
the swiftest traveller being the well-known greenish globe in
Draco,[1532] found to be hurrying towards the earth at the rate of forty
miles a second. For the Orion nebula, a recession of about eleven miles
was determined,[1533] the whole of which may, however, very well belong
to the solar system itself, which, by its translation towards the
constellation Lyra, is certainly leaving the great nebula pretty rapidly
behind. The anomaly of seeming nebular fixity has nevertheless been
removed; and the problem of nebular motion has begun to be solved
through the demonstrated possibility of its spectroscopic investigation.

Keeler's were the first trustworthy determinations of radial m