vantage of
doing away with the prismatic colours, yet it wasted a great deal of
light; for the objection in this respect with regard to loss of light by
reflection from the large mirror applied, of course, to the small mirror
also. In addition, the position of the "flat," as the small mirror is
called, had the further effect of excluding from the great mirror a
certain proportion of light. But the reflector had the advantage, on the
other hand, of costing less to make than the refractor, as it was not
necessary to procure flawless glass for the purpose. A disc of a certain
metallic composition, an alloy of copper and tin, known in consequence
as _speculum metal_, had merely to be cast; and this had to be ground
and polished _upon one side only_, whereas a lens has to be thus treated
_upon both its sides_. It was, therefore, possible to make a much larger
instrument at a great deal less labour and expense.

[Illustration: PLATE III. A TUBELESS, OR "AERIAL" TELESCOPE

From an illustration in the _Opera Varia_ of Christian Huyghens.

(Page 110)]

[Illustration: FIG. 8.--The various types of Telescope. All the above
telescopes are _pointed_ in the same direction; that is to say, the rays
of light from the object are coming from the left-hand side.]

We have given the Newtonian form as an example of the principle of the
reflecting telescope. A somewhat similar instrument had, however, been
projected, though not actually constructed, by James Gregory a few years
earlier than Newton's, _i.e._ in 1663. In this form of reflector, known
as the "Gregorian" telescope, a hole was made in the big concave mirror;
and a small mirror, also concave, which faced it at a certain distance,
received the reflected rays, and reflected them back again through the
hole in question into the eye-piece, which was fixed just behind (see
Fig. 8, p. 113, "Gregorian"). The Gregorian had thus the sentimental
advantage of being _pointed directly at the object_. The hole in the big
mirror did not cause any loss of light, for the central portion in which
it was made was anyway unable to receive light through the small mirror
being directly in front of it. An adaptation of the Gregorian was the
"Cassegrainian" telescope, devised by Cassegrain in 1672, which differed
from it chiefly in the small mirror being convex instead of concave (see
Fig. 8, p. 113, "Cassegrainian"). These _direct-view_ forms of the
reflecting telescope were much in vogue about the