red Light of one Prism, and the other in the violet Light of the other.
The Circles were each of them an Inch in diameter, and behind them the
Wall was dark, that the Experiment might not be disturbed by any Light
coming from thence. These Circles thus illuminated, I viewed through a
Prism, so held, that the Refraction might be made towards the red
Circle, and as I went from them they came nearer and nearer together,
and at length became co-incident; and afterwards when I went still
farther off, they parted again in a contrary Order, the violet by a
greater Refraction being carried beyond the red.
_Exper._ 8. In Summer, when the Sun's Light uses to be strongest, I
placed a Prism at the Hole of the Window-shut, as in the third
Experiment, yet so that its Axis might be parallel to the Axis of the
World, and at the opposite Wall in the Sun's refracted Light, I placed
an open Book. Then going six Feet and two Inches from the Book, I placed
there the above-mentioned Lens, by which the Light reflected from the
Book might be made to converge and meet again at the distance of six
Feet and two Inches behind the Lens, and there paint the Species of the
Book upon a Sheet of white Paper much after the manner of the second
Experiment. The Book and Lens being made fast, I noted the Place where
the Paper was, when the Letters of the Book, illuminated by the fullest
red Light of the Solar Image falling upon it, did cast their Species on
that Paper most distinctly: And then I stay'd till by the Motion of the
Sun, and consequent Motion of his Image on the Book, all the Colours
from that red to the middle of the blue pass'd over those Letters; and
when those Letters were illuminated by that blue, I noted again the
Place of the Paper when they cast their Species most distinctly upon it:
And I found that this last Place of the Paper was nearer to the Lens
than its former Place by about two Inches and an half, or two and three
quarters. So much sooner therefore did the Light in the violet end of
the Image by a greater Refraction converge and meet, than the Light in
the red end. But in trying this, the Chamber was as dark as I could make
it. For, if these Colours be diluted and weakned by the Mixture of any
adventitious Light, the distance between the Places of the Paper will
not be so great. This distance in the second Experiment, where the
Colours of natural Bodies were made use of, was but an Inch and an half,
by reason of the Imperfection of those Colours. Here in the Colours of
the Prism, which are manifestly more full, intense, and lively than
those of natural Bodies, the distance is two Inches and three quarters.
And were the Colours still more full, I question not but that the
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