The year 1668 may be regarded as the date of the invention of
Newton’s Reflecting Telescope. Five years previously, James Gregory
had described the manner of constructing a reflecting telescope with
two concave specula; but Newton perceived the disadvantages to be so
great, that, according to his statement, he “found it necessary, before
attempting any thing in the practice, to alter the design, and place
the eye-glass at the side of the tube rather than at the middle.” On
this improved principle Newton constructed his telescope, which was
examined by Charles II.; it was presented to the Royal Society near the
end of 1671, and is carefully preserved by that distinguished body,
with the inscription:
“THE FIRST REFLECTING TELESCOPE; INVENTED BY SIR ISAAC NEWTON, AND
MADE WITH HIS OWN HANDS.”
Sir David Brewster describes this telescope as consisting of a concave
metallic speculum, the radius of curvature of which was 12-2/3 or
13 inches, so that “it collected the sun’s rays at the distance of
6-1/3 inches.” The rays reflected by the speculum were received upon
a plane metallic speculum inclined 45° to the axis of the tube, so as
to reflect them to the side of the tube in which there was an aperture
to receive a small tube with a plano-convex eye-glass whose radius
was one-twelfth of an inch, by means of which the image formed by
the speculum was magnified 38 times. Such was the first reflecting
telescope applied to the heavens; but Sir David Brewster describes
this instrument as small and ill-made; and fifty years elapsed before
telescopes of the Newtonian form became useful in astronomy.