engthened the current of
prevalent opinion. It is now certain that the evidence furnished on both
sides of the Atlantic as to the stellar composition of some conspicuous
objects of this class (notably the Orion and "Dumb-bell" nebulae) was
delusive; but the spectroscope alone was capable of meeting it with a
categorical denial. Meanwhile there seemed good ground for the
persuasion, which now, for the last time, gained the upper hand, that
nebulae are, without exception, true "island-universes," or assemblages
of distant suns.

Lord Rosse's telescope possesses a nominal power of 6,000--that is, it
shows the moon as if viewed with the naked eye at a distance of forty
miles. But this seeming advantage is neutralised by the weakening of the
available light through excessive diffusion, as well as by the troubles
of the surging sea of air through which the observation must necessarily
be made. Professor Newcomb, in fact, doubts whether with _any_ telescope
our satellite has ever been seen to such advantage as it would be if
brought within 500 miles of the unarmed eye.[334]

The French opticians' rule of doubling the number of millimetres
contained in the aperture of an instrument to find the highest
magnifying power usually applicable to it, would give 3,600 as the
maximum for the leviathan of Birr Castle; but in a climate like that of
Ireland the occasions must be rare when even that limit can be reached.
Indeed, the experience acquired by its use plainly shows that
atmospheric rather than mechanical difficulties impede a still further
increase of telescopic power. Its construction may accordingly be said
to mark the _ne plus ultra_ of effort in one direction, and the
beginning of its conversion towards another. It became thenceforward
more and more obvious that the conditions of observation must be
ameliorated before any added efficacy could be given to it. The full
effect of an uncertain climate in nullifying optical improvements was
recognised, and the attention of astronomers began to be turned towards
the advantages offered by more tranquil and more translucent skies.

Scarcely less important for the practical uses of astronomy than the
optical qualities of the telescope is the manner of its mounting. The
most admirable performance of the optician can render but unsatisfactory
service if its mechanical accessories are ill-arranged or inconvenient.
Thus the astronomer is ultimately dependent upon the mechanician; and s