e same in
principle as his "optick tube," but it is not quite the same in
construction. The early _object-glass_, or large glass at the end, was a
single convex lens (see Fig. 8, p. 113, "Galilean"); the modern one is,
on the other hand, composed of two lenses fitted together. The attempts
to construct large telescopes of the Galilean type met in course of time
with a great difficulty. The magnified image of the object observed was
not quite pure; its edges, indeed, were fringed with rainbow-like
colours. This defect was found to be aggravated with increase in the
size of object-glasses. A method was, however, discovered of
diminishing this colouration, or _chromatic aberration_ as it is called
from the Greek word [chroma] (_chroma_), which means colour, viz. by
making telescopes of great length and only a few inches in width. But
the remedy was, in a way, worse than the disease; for telescopes thus
became of such huge proportions as to be too unwieldy for use. Attempts
were made to evade this unwieldiness by constructing them with skeleton
tubes (see Plate II., p. 110), or, indeed, even without tubes at all;
the object-glass in the tubeless or "aerial" telescope being fixed at
the top of a high post, and the _eye-piece_, that small lens or
combination of lenses, which the eye looks directly into, being kept in
line with it by means of a string and manoeuvred about near the ground
(Plate III., p. 112). The idea of a telescope without a tube may appear
a contradiction in terms; but it is not really so, for the tube adds
nothing to the magnifying power of the instrument, and is, in fact, no
more than a mere device for keeping the object-glass and eye-piece in a
straight line, and for preventing the observer from being hindered by
stray lights in his neighbourhood. It goes without saying, of course,
that the image of a celestial object will be more clear and defined when
examined in the darkness of a tube.

The ancients, though they knew nothing of telescopes, had, however,
found out the merit of a tube in this respect; for they employed simple
tubes, blackened on the inside, in order to obtain a clearer view of
distant objects. It is said that Julius Caesar, before crossing the
Channel, surveyed the opposite coast of Britain through a tube of this
kind.

[Illustration: PLATE II. GREAT TELESCOPE OF HEVELIUS

This instrument, 150 feet in length, with a _skeleton_ tube, was
constructed by the celebrated seventeenth centur