MT than the red end _t_ of the other Spectrum _pt_.
I farther caused those two Spectrums PT [in _Fig._ 20.] and MN to become
co-incident in an inverted Order of their Colours, the red end of each
falling on the violet end of the other, as they are represented in the
oblong Figure PTMN; and then viewing them through a Prism DH held
parallel to their Length, they appeared not co-incident, as when view'd
with the naked Eye, but in the form of two distinct Spectrums _pt_ and
_mn_ crossing one another in the middle after the manner of the Letter
X. Which shews that the red of the one Spectrum and violet of the other,
which were co-incident at PN and MT, being parted from one another by a
greater Refraction of the violet to _p_ and _m_ than of the red to _n_
and _t_, do differ in degrees of Refrangibility.
I illuminated also a little Circular Piece of white Paper all over with
the Lights of both Prisms intermixed, and when it was illuminated with
the red of one Spectrum, and deep violet of the other, so as by the
Mixture of those Colours to appear all over purple, I viewed the Paper,
first at a less distance, and then at a greater, through a third Prism;
and as I went from the Paper, the refracted Image thereof became more
and more divided by the unequal Refraction of the two mixed Colours, and
at length parted into two distinct Images, a red one and a violet one,
whereof the violet was farthest from the Paper, and therefore suffered
the greatest Refraction. And when that Prism at the Window, which cast
the violet on the Paper was taken away, the violet Image disappeared;
but when the other Prism was taken away the red vanished; which shews,
that these two Images were nothing else than the Lights of the two
Prisms, which had been intermixed on the purple Paper, but were parted
again by their unequal Refractions made in the third Prism, through
which the Paper was view'd. This also was observable, that if one of the
Prisms at the Window, suppose that which cast the violet on the Paper,
was turned about its Axis to make all the Colours in this order,
violet, indigo, blue, green, yellow, orange, red, fall successively on
the Paper from that Prism, the violet Image changed Colour accordingly,
turning successively to indigo, blue, green, yellow and red, and in
changing Colour came nearer and nearer to the red Image made by the
other Prism, until when it was also red both Images became fully
co-incident.
I placed also two Paper Circles very near one another, the one in the
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