H. Grubb. One of these, containing a 28-inch object-glass, has been
erected on a mounting originally constructed for a smaller instrument by
Sir G. Airy. The other, presented by Sir Henry Thompson, is of 26 inches
aperture, and is adapted for photographic work.

There is a limit to the size of the refractor depending upon the
material of the object-glass. Glass manufacturers seem to experience
unusual difficulties in their attempts to form large discs of optical
glass pure enough and uniform enough to be suitable for telescopes.
These difficulties are enhanced with every increase in the size of the
discs, so that the cost has a tendency to increase at a very much
greater rate. It may be mentioned in illustration that the price paid
for the object-glass of the Lick telescope exceeded ten thousand pounds.

There is, however, an alternative method of constructing a telescope, in
which the difficulty we have just mentioned does not arise. The
principle of the simplest form of _reflector_ is shown in Fig. 5, which
represents what is called the Herschelian instrument. The rays of light
from the star under observation fall on a mirror which is both carefully
shaped and highly polished. After reflection, the rays proceed to a
focus, and diverging from thence, fall on the eye-piece, by which they
are restored to parallelism, and thus become adapted for reception in
the eye. It was essentially on this principle (though with a secondary
flat mirror at the upper end of the tube reflecting the rays at a right
angle to the side of the tube, where the eye-piece is placed) that Sir
Isaac Newton constructed the little reflecting telescope which is now
treasured by the Royal Society. A famous instrument of the Newtonian
type was built, half a century ago, by the late Earl of Rosse, at
Parsonstown. It is represented in Fig. 7. The colossal aperture of this
instrument has never been surpassed; it has, indeed, never been
rivalled. The mirror or speculum, as it is often called, is a thick
metallic disc, composed of a mixture of two parts of copper with one of
tin. This alloy is so hard and brittle as to make the necessary
mechanical operations difficult to manage. The material admits, however,
of a brilliant polish, and of receiving and retaining an accurate
figure. The Rosse speculum--six feet in diameter and three tons in
weight--reposes at the lower end of a telescope fifty-five feet long.
The tube is suspended between two massive castel