[Illustration: FIG. 27.]
But you will say, if the Errors caused by the different Refrangibility
be so very great, how comes it to pass, that Objects appear through
Telescopes so distinct as they do? I answer, 'tis because the erring
Rays are not scattered uniformly over all that Circular Space, but
collected infinitely more densely in the Center than in any other Part
of the Circle, and in the Way from the Center to the Circumference, grow
continually rarer and rarer, so as at the Circumference to become
infinitely rare; and by reason of their Rarity are not strong enough to
be visible, unless in the Center and very near it. Let ADE [in _Fig._
27.] represent one of those Circles described with the Center C, and
Semi-diameter AC, and let BFG be a smaller Circle concentrick to the
former, cutting with its Circumference the Diameter AC in B, and bisect
AC in N; and by my reckoning, the Density of the Light in any Place B,
will be to its Density in N, as AB to BC; and the whole Light within the
lesser Circle BFG, will be to the whole Light within the greater AED, as
the Excess of the Square of AC above the Square of AB, is to the Square
of AC. As if BC be the fifth Part of AC, the Light will be four times
denser in B than in N, and the whole Light within the less Circle, will
be to the whole Light within the greater, as nine to twenty-five. Whence
it's evident, that the Light within the less Circle, must strike the
Sense much more strongly, than that faint and dilated Light round about
between it and the Circumference of the greater.
But it's farther to be noted, that the most luminous of the Prismatick
Colours are the yellow and orange. These affect the Senses more strongly
than all the rest together, and next to these in strength are the red
and green. The blue compared with these is a faint and dark Colour, and
the indigo and violet are much darker and fainter, so that these
compared with the stronger Colours are little to be regarded. The Images
of Objects are therefore to be placed, not in the Focus of the mean
refrangible Rays, which are in the Confine of green and blue, but in the
Focus of those Rays which are in the middle of the orange and yellow;
there where the Colour is most luminous and fulgent, that is in the
brightest yellow, that yellow which inclines more to orange than to
green. And by the Refraction of these Rays (whose Sines of Incidence and
Refraction in Glass are as 17 and 11) the Refraction of Glass and
Crystal for Optical Uses is to be measured. Let us therefore place the
56