e it returns into Sight all at
once, or certainly in less time than the second of a Minute; the
Refraction of the Moon's Atmosphere a little protracting the time in
which the Light of the Star first vanishes, and afterwards returns into
Sight.

Now, if we suppose the sensible Image of a lucid Point, to be even 250
times narrower than the Aperture of the Glass; yet this Image would be
still much greater than if it were only from the spherical Figure of the
Glass. For were it not for the different Refrangibility of the Rays, its
breadth in an 100 Foot Telescope whose aperture is 4 Inches, would be
but 961/72000000 parts of an Inch, as is manifest by the foregoing
Computation. And therefore in this case the greatest Errors arising from
the spherical Figure of the Glass, would be to the greatest sensible
Errors arising from the different Refrangibility of the Rays as
961/72000000 to 4/250 at most, that is only as 1 to 1200. And this
sufficiently shews that it is not the spherical Figures of Glasses, but
the different Refrangibility of the Rays which hinders the perfection of
Telescopes.

There is another Argument by which it may appear that the different
Refrangibility of Rays, is the true cause of the imperfection of
Telescopes. For the Errors of the Rays arising from the spherical
Figures of Object-glasses, are as the Cubes of the Apertures of the
Object Glasses; and thence to make Telescopes of various Lengths magnify
with equal distinctness, the Apertures of the Object-glasses, and the
Charges or magnifying Powers ought to be as the Cubes of the square
Roots of their lengths; which doth not answer to Experience. But the
Errors of the Rays arising from the different Refrangibility, are as the
Apertures of the Object-glasses; and thence to make Telescopes of
various lengths, magnify with equal distinctness, their Apertures and
Charges ought to be as the square Roots of their lengths; and this
answers to Experience, as is well known. For Instance, a Telescope of 64
Feet in length, with an Aperture of 2-2/3 Inches, magnifies about 120
times, with as much distinctness as one of a Foot in length, with 1/3 of
an Inch aperture, magnifies 15 times.

[Illustration: FIG. 28.]

Now were it not for this different Refrangibility of Rays, Telescopes
might be brought to a greater perfection than we have yet describ'd, by
composing the Object-glass of two Glasses with Water between them. Let
ADFC [in _Fig._ 28.] represent the Objec