to green, I took another Prism of clear white Glass; but the Spectrum of
Colours which this Prism made had long white Streams of faint Light
shooting out from both ends of the Colours, which made me conclude that
something was amiss; and viewing the Prism, I found two or three little
Bubbles in the Glass, which refracted the Light irregularly. Wherefore I
covered that Part of the Glass with black Paper, and letting the Light
pass through another Part of it which was free from such Bubbles, the
Spectrum of Colours became free from those irregular Streams of Light,
and was now such as I desired. But still I found the violet so dark and
faint, that I could scarce see the Species of the Lines by the violet,
and not at all by the deepest Part of it, which was next the end of the
Spectrum. I suspected therefore, that this faint and dark Colour might
be allayed by that scattering Light which was refracted, and reflected
irregularly, partly by some very small Bubbles in the Glasses, and
partly by the Inequalities of their Polish; which Light, tho' it was but
little, yet it being of a white Colour, might suffice to affect the
Sense so strongly as to disturb the Ph nomena of that weak and dark
Colour the violet, and therefore I tried, as in the 12th, 13th, and 14th
Experiments, whether the Light of this Colour did not consist of a
sensible Mixture of heterogeneous Rays, but found it did not. Nor did
the Refractions cause any other sensible Colour than violet to emerge
out of this Light, as they would have done out of white Light, and by
consequence out of this violet Light had it been sensibly compounded
with white Light. And therefore I concluded, that the reason why I could
not see the Species of the Lines distinctly by this Colour, was only
the Darkness of this Colour, and Thinness of its Light, and its distance
from the Axis of the Lens; I divided therefore those Parallel black
Lines into equal Parts, by which I might readily know the distances of
the Colours in the Spectrum from one another, and noted the distances of
the Lens from the Foci of such Colours, as cast the Species of the Lines
distinctly, and then considered whether the difference of those
distances bear such proportion to 5-1/3 Inches, the greatest Difference
of the distances, which the Foci of the deepest red and violet ought to
have from the Lens, as the distance of the observed Colours from one
another in the Spectrum bear to the greatest distance of the deepest red
and violet measured in the Rectilinear Sides of the Spectrum, that is,
to the Length of those Sides, or Excess of the Length of the Spectrum
above its Breadth. And my Observations were as follows.
When I observed and compared the deepest sensible red, and the Colour in
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