six Feet and an Inch from the Lens on the other side. And therefore by
the foregoing Rule, it ought to collect the Species of that Object by
the least refrangible Rays at the distance of six Feet and 3-2/3 Inches
from the Lens, and by the most refrangible ones at the distance of five
Feet and 10-1/3 Inches from it: So that between the two Places, where
these least and most refrangible Rays collect the Species, there may be
the distance of about 5-1/3 Inches. For by that Rule, as six Feet and an
Inch (the distance of the Lens from the lucid Object) is to twelve Feet
and two Inches (the distance of the lucid Object from the Focus of the
mean refrangible Rays) that is, as One is to Two; so is the 27-1/2th
Part of six Feet and an Inch (the distance between the Lens and the same
Focus) to the distance between the Focus of the most refrangible Rays
and the Focus of the least refrangible ones, which is therefore 5-17/55
Inches, that is very nearly 5-1/3 Inches. Now to know whether this
Measure was true, I repeated the second and eighth Experiment with
coloured Light, which was less compounded than that I there made use of:
For I now separated the heterogeneous Rays from one another by the
Method I described in the eleventh Experiment, so as to make a coloured
Spectrum about twelve or fifteen Times longer than broad. This Spectrum
I cast on a printed Book, and placing the above-mentioned Lens at the
distance of six Feet and an Inch from this Spectrum to collect the
Species of the illuminated Letters at the same distance on the other
side, I found that the Species of the Letters illuminated with blue were
nearer to the Lens than those illuminated with deep red by about three
Inches, or three and a quarter; but the Species of the Letters
illuminated with indigo and violet appeared so confused and indistinct,
that I could not read them: Whereupon viewing the Prism, I found it was
full of Veins running from one end of the Glass to the other; so that
the Refraction could not be regular. I took another Prism therefore
which was free from Veins, and instead of the Letters I used two or
three Parallel black Lines a little broader than the Strokes of the
Letters, and casting the Colours upon these Lines in such manner, that
the Lines ran along the Colours from one end of the Spectrum to the
other, I found that the Focus where the indigo, or confine of this
Colour and violet cast the Species of the black Lines most distinctly,
to be about four Inches, or 4-1/4 nearer to the Lens than the Focus,
where the deepest red cast the Species of the same black Lines most
distinctly. The violet was so faint and dark, that I could not discern
the Species of the Lines distinctly by that Colour; and therefore
considering that the Prism was made of a dark coloured Glass inclining
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