Sir,

To perform my late promise to you, I shall without further
ceremony acquaint you that in the beginning of the year 1666 (at
which time I applied myself to the grinding of optic glasses of
other figures than spherical) I procured me a triangular glass
prism, to try therewith the celebrated phenomena of colors. And
in order thereto having darkened my chamber and made a small
hole in my window shuts, to let in a convenient quantity of the
sun's light, I placed my prism at his entrance, that it might be
thereby refracted to the opposite wall. It was at first a pleasing
divertisement to view the vivid and intense colors produced
thereby; but after a while applying myself to consider them more
circumspectly, I became surprised to see them in an oblong form;
which, according to the received laws of refraction, I expected
should have been circular.

They were terminated at the sides with straight lines, but at
the ends the decay of light was so gradual that it was difficult to
determine justly what was their figure; yet they seemed semi-
circular.

Comparing the length of this colored spectrum with its breadth,
I found it about five times greater, a disproportion so extrava-
gant that it excited me to a more than ordinary curiosity of
examining from whence it might proceed. I could scarce think
that the various thickness of the glass, or the termination with
shadow or darkness, could have any influence on light to produce

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