above, I conclude, that it was made by Refraction in some sort of Hail
or Snow floating in the Air in an horizontal posture, the refracting
Angle being about 58 or 60 Degrees.
THE
THIRD BOOK
OF
OPTICKS
_PART I._
_Observations concerning the Inflexions of the Rays of Light, and the
Colours made thereby._
Grimaldo has inform'd us, that if a beam of the Sun's Light be let into
a dark Room through a very small hole, the Shadows of things in this
Light will be larger than they ought to be if the Rays went on by the
Bodies in straight Lines, and that these Shadows have three parallel
Fringes, Bands or Ranks of colour'd Light adjacent to them. But if the
Hole be enlarged the Fringes grow broad and run into one another, so
that they cannot be distinguish'd. These broad Shadows and Fringes have
been reckon'd by some to proceed from the ordinary refraction of the
Air, but without due examination of the Matter. For the circumstances of
the Ph nomenon, so far as I have observed them, are as follows.
_Obs._ 1. I made in a piece of Lead a small Hole with a Pin, whose
breadth was the 42d part of an Inch. For 21 of those Pins laid together
took up the breadth of half an Inch. Through this Hole I let into my
darken'd Chamber a beam of the Sun's Light, and found that the Shadows
of Hairs, Thred, Pins, Straws, and such like slender Substances placed
in this beam of Light, were considerably broader than they ought to be,
if the Rays of Light passed on by these Bodies in right Lines. And
particularly a Hair of a Man's Head, whose breadth was but the 280th
180