l telescope must be specially
borne in mind with regard to the photographs of the moon in Chapter XVI.

In the year 1825 the largest achromatic refractor in existence was one
of nine and a half inches in diameter constructed by Fraunhofer for the
Observatory of Dorpat in Russia. The largest refractors in the world
to-day are in the United States, _i.e._ the forty-inch of the Yerkes
Observatory (see Plate IV., p. 118), and the thirty-six inch of the
Lick. The object-glasses of these and of the thirty-inch telescope of
the Observatory of Pulkowa, in Russia, were made by the great optical
house of Alvan Clark & Sons, of Cambridge, Massachusetts, U.S.A. The
tubes and other portions of the Yerkes and Lick telescopes were,
however, constructed by the Warner and Swasey Co., of Cleveland, Ohio.

The largest reflector, and so the largest telescope in the world, is
still the six-foot erected by the late Lord Rosse at Parsonstown in
Ireland, and completed in the year 1845. It is about fifty-six feet in
length. Next come two of five feet, with mirrors of silver on glass;
one of them made by the late Dr. Common, of Ealing, and the other by the
American astronomer, Professor G.W. Ritchey. The latter of these is
installed in the Solar Observatory belonging to Carnegie Institution of
Washington, which is situated on Mount Wilson in California. The former
is now at the Harvard College Observatory, and is considered by
Professor Moulton to be probably the most efficient reflector in use at
present. Another large reflector is the three-foot made by Dr. Common.
It came into the possession of Mr. Crossley of Halifax, who presented it
to the Lick Observatory, where it is now known as the "Crossley
Reflector."

Although to the house of Clark belongs, as we have seen, the credit of
constructing the object-glasses of the largest refracting telescopes of
our time, it has nevertheless keen competitors in Sir Howard Grubb, of
Dublin, and such well-known firms as Cooke of York and Steinheil of
Munich. In the four-foot reflector, made in 1870 for the Observatory of
Melbourne by the firm of Grubb, the Cassegrainian principle was
employed.

With regard to the various merits of refractors and reflectors much
might be said. Each kind of instrument has, indeed, its special
advantages; though perhaps, on the whole, the most perfect type of
telescope is the achromatic refractor.

[Illustration: PLATE IV. THE GREAT YERKES TELESCOPE

Great telescope a