middle of the
eighteenth century, when many beautiful examples of Gregorians were made
by the famous optician, James Short, of Edinburgh.

An adaptation of the Newtonian type of telescope is known as the
"Herschelian," from being the kind favoured by Sir William Herschel. It
is, however, only suitable in immense instruments, such as Herschel was
in the habit of employing. In this form the object-glass is set at a
slight slant, so that the light coming from the object is reflected
straight into the eye-piece, which is fixed facing it in the side of the
tube (see Fig. 8, p. 113, "Herschelian"). This telescope has an
advantage over the other forms of reflector through the saving of light
consequent on doing away with the _second_ reflection. There is,
however, the objection that the slant of the object-glass is productive
of some distortion in the appearance of the object observed; but this
slant is of necessity slight when the length of the telescope is very
great.

The principle of this type of telescope had been described to the
French Academy of Sciences as early as 1728 by Le Maire, but no one
availed himself of the idea until 1776, when Herschel tried it. At
first, however, he rejected it; but in 1786 he seems to have found that
it suited the huge instruments which he was then making. Herschel's
largest telescope, constructed in 1789, was about four feet in diameter
and forty feet in length. It is generally spoken of as the "Forty-foot
Telescope," though all other instruments have been known by their
_diameters_, rather than by their lengths.

To return to the refracting telescope. A solution of the colour
difficulty was arrived at in 1729 (two years after Newton's death) by an
Essex gentleman named Chester Moor Hall. He discovered that by making a
double object-glass, composed of an outer convex lens and an inner
concave lens, made respectively of different kinds of glass, _i.e._
_crown_ glass and _flint_ glass, the troublesome colour effects could
be, _to a very great extent_, removed. Hall's investigations appear to
have been rather of an academic nature; and, although he is believed to
have constructed a small telescope upon these lines, yet he seems to
have kept the matter so much to himself that it was not until the year
1758 that the first example of the new instrument was given to the
world. This was done by John Dollond, founder of the well-known optical
firm of Dollond, of Ludgate Hill, London, who had,