Image of the Object in the Focus of these Rays, and all the yellow and
orange will fall within a Circle, whose Diameter is about the 250th
Part of the Diameter of the Aperture of the Glass. And if you add the
brighter half of the red, (that half which is next the orange) and the
brighter half of the green, (that half which is next the yellow) about
three fifth Parts of the Light of these two Colours will fall within the
same Circle, and two fifth Parts will fall without it round about; and
that which falls without will be spread through almost as much more
space as that which falls within, and so in the gross be almost three
times rarer. Of the other half of the red and green, (that is of the
deep dark red and willow green) about one quarter will fall within this
Circle, and three quarters without, and that which falls without will be
spread through about four or five times more space than that 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
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