lf 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
part of an Inch, being held in this Light, at the distance of about
twelve Feet from the Hole, did cast a Shadow which at the distance of
four Inches from the Hair was the sixtieth part of an Inch broad, that
is, above four times broader than the Hair, and at the distance of two
Feet from the Hair was about the eight and twentieth part of an Inch
broad, that is, ten times broader than the Hair, and at the distance of
ten Feet was the eighth part of an Inch broad, that is 35 times broader.

Nor is it material whether the Hair be encompassed with Air, or with any
other pellucid Substance. For I wetted a polish'd Plate of Glass, and
laid the Hair in the Water upon the Glass, and then laying another
polish'd Plate of Glass upon it, so that the Water might fill up the
space between the Glasses, I held them in the aforesaid beam of Light,
so that the Light might pass through them perpendicularly, and the
Shadow of the Hair was at the same distances as big as before. The
Shadows of Scratches made in polish'd Plates of Glass were also much
broader than they ought to be, and the Veins in polish'd Plates of Glass
did also cast the like broad Shadows. And therefore the great breadth of
these Shadows proceeds from some other cause than the Refraction of the
Air.

Let the Circle X [in _Fig._ 1.] represent the middle of the Hair; ADG,
BEH, CFI, three Rays passing by one side of the Hair at several
distances; KNQ, LOR, MPS, three other Rays passing by the other side of
the Hair at the like distances; D, E, F, and N, O, P, the places where
the Rays are bent in their passage by the Hair; G, H, I, and Q, R, S,
the places where the Rays fall on a Paper GQ; IS the breadth of the
Shadow of the Hair cast on the Paper, and TI, VS, two Rays passing to
the Points I and S without bending when the Hair is taken away. And it's
manifest that all the Light between these two Rays TI and VS is bent in
passing by the Hair, and turned aside from the Shadow IS, because if any
part of this Light were not bent it would fall on the Paper within the
Shadow, and there ill