which falls
within; and so in the gross be rarer, and if compared with the whole
Light within it, will be about 25 times rarer than all that taken in the
gross; or rather more than 30 or 40 times rarer, because the deep red in
the end of the Spectrum of Colours made by a Prism is very thin and
rare, and the willow green is something rarer than the orange and
yellow. The Light of these Colours therefore being so very much rarer
than that within the Circle, will scarce affect the Sense, especially
since the deep red and willow green of this Light, are much darker
Colours than the rest. And for the same reason the blue and violet being
much darker Colours than these, and much more rarified, may be
neglected. For the dense and bright Light of the Circle, will obscure
the rare and weak Light of these dark Colours round about it, and
render them almost insensible. The sensible Image of a lucid Point is
therefore scarce broader than a Circle, whose Diameter is the 250th Part
of the Diameter of the Aperture of the Object-glass of a good Telescope,
or not much broader, if you except a faint and dark misty Light round
about it, which a Spectator will scarce regard. And therefore in a
Telescope, whose Aperture is four Inches, and Length an hundred Feet, it
exceeds not 2'' 45''', or 3''. And in a Telescope whose Aperture is two
Inches, and Length 20 or 30 Feet, it may be 5'' or 6'', and scarce
above. And this answers well to Experience: For some Astronomers have
found the Diameters of the fix'd Stars, in Telescopes of between 20 and
60 Feet in length, to be about 5'' or 6'', or at most 8'' or 10'' in
diameter. But if the Eye-Glass be tincted faintly with the Smoak of a
Lamp or Torch, to obscure the Light of the Star, the fainter Light in
the Circumference of the Star ceases to be visible, and the Star (if the
Glass be sufficiently soiled with Smoak) appears something more like a
mathematical Point. And for the same Reason, the enormous Part of the
Light in the Circumference of every lucid Point ought to be less
discernible in shorter Telescopes than in longer, because the shorter
transmit less Light to the Eye.

Now, that the fix'd Stars, by reason of their immense Distance, appear
like Points, unless so far as their Light is dilated by Refraction, may
appear from hence; that when the Moon passes over them and eclipses
them, their Light vanishes, not gradually like that of the Planets, but
all at once; and in the end of the Eclips