and easily number eight or nine of them, and sometimes twelve or
thirteen. And had not their Light been so very faint, I question not but
that I might have seen many more.
_Obs._ 5. Placing a Prism at the Window to refract the intromitted beam
of Light, and cast the oblong Spectrum of Colours on the Speculum: I
covered the Speculum with a black Paper which had in the middle of it a
hole to let any one of the Colours pass through to the Speculum, whilst
the rest were intercepted by the Paper. And now I found Rings of that
Colour only which fell upon the Speculum. If the Speculum was
illuminated with red, the Rings were totally red with dark Intervals, if
with blue they were totally blue, and so of the other Colours. And when
they were illuminated with any one Colour, the Squares of their
Diameters measured between their most luminous Parts, were in the
arithmetical Progression of the Numbers, 0, 1, 2, 3, 4 and the Squares
of the Diameters of their dark Intervals in the Progression of the
intermediate Numbers 1/2, 1-1/2, 2-1/2, 3-1/2. But if the Colour was
varied, they varied their Magnitude. In the red they were largest, in
the indigo and violet least, and in the intermediate Colours yellow,
green, and blue, they were of several intermediate Bignesses answering
to the Colour, that is, greater in yellow than in green, and greater in
green than in blue. And hence I knew, that when the Speculum was
illuminated with white Light, the red and yellow on the outside of the
Rings were produced by the least refrangible Rays, and the blue and
violet by the most refrangible, and that the Colours of each Ring spread
into the Colours of the neighbouring Rings on either side, after the
manner explain'd in the first and second Part of this Book, and by
mixing diluted one another so that they could not be distinguish'd,
unless near the Center where they were least mix'd. For in this
Observation I could see the Rings more distinctly, and to a greater
Number than before, being able in the yellow Light to number eight or
nine of them, besides a faint shadow of a tenth. To satisfy my self how
much the Colours of the several Rings spread into one another, I
measured the Diameters of the second and third Rings, and found them
when made by the Confine of the red and orange to be to the same
Diameters when made by the Confine of blue and indigo, as 9 to 8, or
thereabouts. For it was hard to determine this Proportion accurately.
Also the Circles made successively by the red, yellow, and green,
differ'd more from one another than those made successively by the
green, blue, and indigo. For the Circle made by the violet was too dark
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