om and promising it to each of the sciences,
and patiently waiting upon the advances of all. The science of the
heavenly bodies has, in a word, become a branch of terrestrial physics,
or rather a higher kind of integration of all their results. It has,
however, this leading peculiarity, that the materials for the whole of
its inquiries are telescopically furnished. They are such as come very
imperfectly, or not at all, within the cognisance of the unarmed eye.

Spectroscopic and photographic apparatus are simply additions to the
telescope. They do not supersede or render it of less importance. On the
contrary, the efficacy of their action depends primarily upon the
optical qualities of the instruments they are attached to. Hence the
development, to their fullest extent, of the powers of the telescope is
of vital moment to the progress of modern physical astronomy, while the
older mathematical astronomy could afford to remain comparatively
indifferent to it.

The colossal Rosse reflector still marks, as to size, the _ne plus
ultra_ of performance in that line. A mirror four feet in diameter was,
however, sent out to Melbourne by the late Thomas Grubb of Dublin in
1870. This is mounted in the Cassegrainian manner, so that the observer
looks straight through it towards the object viewed, of which he really
sees a twice-reflected image. The dust-laden atmosphere of Melbourne is
said to impede very seriously the usefulness of this originally fine
instrument.

It may be doubted whether so large a spectrum will ever again be
constructed. A new material for the mirrors of reflecting telescopes,
proposed by Steinheil in 1856, and independently by Foucault in
1857,[1630] has in a great measure superseded the use of a metallic
alloy. This is glass upon which a thin film of silver has been deposited
by a chemical process originally invented by Liebig. It gives a
peculiarly brilliant reflective surface, throwing back more light than a
metallic mirror of the same area, in the proportion of about sixteen to
nine. Resilvering, too, involves much less risk and trouble than
repolishing a speculum. The first use of this plan on a large scale was
in an instrument of thirty-six inches aperture, finished by Calver for
Dr. Common in 1879. To its excellent qualities turned to account with
rare skill, his triumphs in celestial photography are mainly due. A more
daring experiment was the construction and mounting, by Dr. Common
himself, of a