t the Yerkes Observatory of the University of Chicago,
Williams Bay, Wisconsin, U.S.A. It was erected in 1896-7, and is the
largest refracting telescope in the world. Diameter of object-glass, 40
inches; length of telescope, about 60 feet. The object-glass was made by
the firm of Alvan Clark and Sons, of Cambridge, Massachusetts; the other
portions of the instrument by the Warner and Swasey Co., of Cleveland,
Ohio.

(Page 117)]

In connection with telescopes certain devices have from time to time
been introduced, but these merely aim at the _convenience_ of the
observer and do not supplant the broad principles upon which are based
the various types of instrument above described. Such, for instance, are
the "Siderostat," and another form of it called the "Coelostat," in
which a plane mirror is made to revolve in a certain manner, so as to
reflect those portions of the sky which are to be observed, into the
tube of a telescope kept fixed. Such too are the "Equatorial Coude" of
the late M. Loewy, Director of the Paris Observatory, and the
"Sheepshanks Telescope" of the Observatory of Cambridge, in which a
telescope is separated into two portions, the eye-piece portion being
fixed upon a downward slant, and the object-glass portion jointed to it
at an angle and pointed up at the sky. In these two instruments (which,
by the way, differ materially) an arrangement of slanting mirrors in the
tubes directs the journey of the rays of light from the object-glass to
the eye-piece. The observer can thus sit at the eye-end of his telescope
in the warmth and comfort of his room, and observe the stars in the same
unconstrained manner as if he were merely looking down into a
microscope.

Needless to say, devices such as these are subject to the drawback that
the mirrors employed sap a certain proportion of the rays of light. It
will be remembered that we made allusion to loss of light in this way,
when pointing out the advantage in light grasp of the Herschelian form
of telescope, where only _one_ reflection takes place, over the
Newtonian in which there are _two_.

It is an interesting question as to whether telescopes can be made much
larger. The American astronomer, Professor G.E. Hale, concludes that the
limit of refractors is about five feet in diameter, but he thinks that
reflectors as large as nine feet in diameter might now be made. As
regards refractors there are several strong reasons against augmenting
their proportions.