[Illustration: FIG. 11.]
_Exper._ 2. About the aforesaid Paper, whose two halfs were painted over
with red and blue, and which was stiff like thin Pasteboard, I lapped
several times a slender Thred of very black Silk, in such manner that
the several parts of the Thred might appear upon the Colours like so
many black Lines drawn over them, or like long and slender dark Shadows
cast upon them. I might have drawn black Lines with a Pen, but the
Threds were smaller and better defined. This Paper thus coloured and
lined I set against a Wall perpendicularly to the Horizon, so that one
of the Colours might stand to the Right Hand, and the other to the Left.
Close before the Paper, at the Confine of the Colours below, I placed a
Candle to illuminate the Paper strongly: For the Experiment was tried in
the Night. The Flame of the Candle reached up to the lower edge of the
Paper, or a very little higher. Then at the distance of six Feet, and
one or two Inches from the Paper upon the Floor I erected a Glass Lens
four Inches and a quarter broad, which might collect the Rays coming
from the several Points of the Paper, and make them converge towards so
many other Points at the same distance of six Feet, and one or two
Inches on the other side of the Lens, and so form the Image of the
coloured Paper upon a white Paper placed there, after the same manner
that a Lens at a Hole in a Window casts the Images of Objects abroad
upon a Sheet of white Paper in a dark Room. The aforesaid white Paper,
erected perpendicular to the Horizon, and to the Rays which fell upon it
from the Lens, I moved sometimes towards the Lens, sometimes from it, to
find the Places where the Images of the blue and red Parts of the
coloured Paper appeared most distinct. Those Places I easily knew by the
Images of the black Lines which I had made by winding the Silk about the
Paper. For the Images of those fine and slender Lines (which by reason
of their Blackness were like Shadows on the Colours) were confused and
scarce visible, unless when the Colours on either side of each Line were
terminated most distinctly, Noting therefore, as diligently as I could,
the Places where the Images of the red and blue halfs of the coloured
Paper appeared most distinct, I found that where the red half of the
Paper appeared distinct, the blue half appeared confused, so that the
black Lines drawn upon it could scarce be seen; and on the contrary,
where the blue half appeared most distinct, the red half appeared
confused, so that the black Lines upon it were scarce visible. And
between the two Places where these Images appeared distinct there was
the distance of an Inch and a half; the distance of the white Paper from
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