and therefore the Image would be round. Thus it would be were the two
Rays XLJT and YKHP, and all the rest which form the Image P_w_T_v_,
alike refrangible. And therefore seeing by Experience it is found that
the Image is not round, but about five times longer than broad, the Rays
which going to the upper end P of the Image suffer the greatest
Refraction, must be more refrangible than those which go to the lower
end T, unless the Inequality of Refraction be casual.
This Image or Spectrum PT was coloured, being red at its least refracted
end T, and violet at its most refracted end P, and yellow green and
blue in the intermediate Spaces. Which agrees with the first
Proposition, that Lights which differ in Colour, do also differ in
Refrangibility. The length of the Image in the foregoing Experiments, I
measured from the faintest and outmost red at one end, to the faintest
and outmost blue at the other end, excepting only a little Penumbra,
whose breadth scarce exceeded a quarter of an Inch, as was said above.
_Exper._ 4. In the Sun's Beam which was propagated into the Room through
the hole in the Window-shut, at the distance of some Feet from the hole,
I held the Prism in such a Posture, that its Axis might be perpendicular
to that Beam. Then I looked through the Prism upon the hole, and turning
the Prism to and fro about its Axis, to make the Image of the Hole
ascend and descend, when between its two contrary Motions it seemed
Stationary, I stopp'd the Prism, that the Refractions of both sides of
the refracting Angle might be equal to each other, as in the former
Experiment. In this situation of the Prism viewing through it the said
Hole, I observed the length of its refracted Image to be many times
greater than its breadth, and that the most refracted part thereof
appeared violet, the least refracted red, the middle parts blue, green
and yellow in order. The same thing happen'd when I removed the Prism
out of the Sun's Light, and looked through it upon the hole shining by
the Light of the Clouds beyond it. And yet if the Refraction were done
regularly according to one certain Proportion of the Sines of Incidence
and Refraction as is vulgarly supposed, the refracted Image ought to
have appeared round.
So then, by these two Experiments it appears, that in Equal Incidences
there is a considerable inequality of Refractions. But whence this
inequality arises, whether it be that some of the incident Rays are
refracted more, and others less, constantly, or by chance, or that one
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