quite independently,
re-discovered the principle.

This "Achromatic" telescope, or telescope "free from colour effects," is
the kind ordinarily in use at present, whether for astronomical or for
terrestrial purposes (see Fig. 8, p. 113, "Achromatic"). The expense of
making large instruments of this type is very great, for, in the
object-glass alone, no less than _four_ surfaces have to be ground and
polished to the required curves; and, usually, the two lenses of which
it is composed have to fit quite close together.

With the object of evading the expense referred to, and of securing
_complete_ freedom from colour effects, telescopes have even been made,
the object-glasses of which were composed of various transparent liquids
placed between thin lenses; but leakages, and currents set up within
them by changes of temperature, have defeated the ingenuity of those who
devised these substitutes.

The solution of the colour difficulty by means of Dollond's achromatic
refractor has not, however, ousted the reflecting telescope in its best,
or Newtonian form, for which great concave mirrors made of glass,
covered with a thin coating of silver and highly polished, have been
used since about 1870 instead of metal mirrors. They are very much
lighter in weight and cheaper to make than the old specula; and though
the silvering, needless to say, deteriorates with time, it can be
renewed at a comparatively trifling cost. Also these mirrors reflect
much more light, and give a clearer view, than did the old metallic
ones.

When an object is viewed through the type of astronomical telescope
ordinarily in use, it is seen _upside down_. This is, however, a matter
of very small moment in dealing with celestial objects; for, as they are
usually round, it is really not of much consequence which part we regard
as top and which as bottom. Such an inversion would, of course, be most
inconvenient when viewing terrestrial objects. In order to observe the
latter we therefore employ what is called a terrestrial telescope, which
is merely a refractor with some extra lenses added in the eye portion
for the purpose of turning the inverted image the right way up again.
These extra lenses, needless to say, absorb a certain amount of light;
wherefore it is better in astronomical observation to save light by
doing away with them, and putting up with the slight inconvenience of
seeing the object inverted.

This inversion of images by the astronomica