y astronomer, Hevelius
of Danzig. From an illustration in the _Machina Celestis_.

(Page 110)]

A few of the most famous of the immensely long telescopes above alluded
to are worthy of mention. One of these, 123 feet in length, was
presented to the Royal Society of London by the Dutch astronomer
Huyghens. Hevelius of Danzig constructed a skeleton one of 150 feet in
length (see Plate II., p. 110). Bradley used a tubeless one 212 feet
long to measure the diameter of Venus in 1722; while one of 600 feet is
said to have been constructed, but to have proved quite unworkable!

Such difficulties, however, produced their natural result. They set men
at work to devise another kind of telescope. In the new form, called the
Reflecting Telescope, or "Reflector," the light coming from the object
under observation was _reflected_ into the eye-piece from the surface of
a highly polished concave metallic mirror, or _speculum_, as it was
called. It is to Sir Isaac Newton that the world is indebted for the
reflecting telescope in its best form. That philosopher had set himself
to investigate the causes of the rainbow-like, or prismatic colours
which for a long time had been such a source of annoyance to telescopic
observers; and he pointed out that, as the colours were produced in the
passage of the rays of light _through_ the glass, they would be entirely
absent if the light were reflected from the _surface_ of a mirror
instead.

The reflecting telescope, however, had in turn certain drawbacks of its
own. A mirror, for instance, can plainly never be polished to such a
high degree as to reflect as much light as a piece of transparent glass
will let through. Further, the position of the eye-piece is by no means
so convenient. It cannot, of course, be pointed directly towards the
mirror, for the observer would then have to place his head right in the
way of the light coming from the celestial object, and would thus, of
course, cut it off. In order to obviate this difficulty, the following
device was employed by Newton in his telescope, of which he constructed
his first example in 1668. A small, flat mirror was fixed by thin wires
in the centre of the tube of the telescope, and near to its open end. It
was set slant-wise, so that it reflected the rays of light directly into
the eye-piece, which was screwed into a hole at the side of the tube
(see Fig. 8, p. 113, "Newtonian").

Although the Newtonian form of telescope had the immense ad