Numbers 1, sqrt(1/3), sqrt(1/5), and their Intervals to be in the
same progression with them; that is, the Fringes and their Intervals
together to be in the continual progression of the Numbers 1,
sqrt(1/2), sqrt(1/3), sqrt(1/4), sqrt(1/5), or thereabouts. And
these Proportions held the same very nearly at all distances from the
Hair; the dark Intervals of the Fringes being as broad in proportion to
the breadth of the Fringes at their first appearance as afterwards at
great distances from the Hair, though not so dark and distinct.
_Obs._ 5. The Sun shining into my darken'd Chamber through a hole a
quarter of an Inch broad, I placed at the distance of two or three Feet
from the Hole a Sheet of Pasteboard, which was black'd all over on both
sides, and in the middle of it had a hole about three quarters of an
Inch square for the Light to pass through. And behind the hole I
fasten'd to the Pasteboard with Pitch the blade of a sharp Knife, to
intercept some part of the Light which passed through the hole. The
Planes of the Pasteboard and blade of the Knife were parallel to one
another, and perpendicular to the Rays. And when they were so placed
that none of the Sun's Light fell on the Pasteboard, but all of it
passed through the hole to the Knife, and there part of it fell upon the
blade of the Knife, and part of it passed by its edge; I let this part
of the Light which passed by, fall on a white Paper two or three Feet
beyond the Knife, and there saw two streams of faint Light shoot out
both ways from the beam of Light into the shadow, like the Tails of
Comets. But because the Sun's direct Light by its brightness upon the
Paper obscured these faint streams, so that I could scarce see them, I
made a little hole in the midst of the Paper for that Light to pass
through and fall on a black Cloth behind it; and then I saw the two
streams plainly. They were like one another, and pretty nearly equal in
length, and breadth, and quantity of Light. Their Light at that end next
the Sun's direct Light was pretty strong for the space of about a
quarter of an Inch, or half an Inch, and in all its progress from that
direct Light decreased gradually till it became insensible. The whole
length of either of these streams measured upon the paper at the
distance of three Feet from the Knife was about six or eight Inches; so
that it subtended an Angle at the edge of the Knife of about 10 or 12,
or at most 14 Degrees. Yet sometimes I thought I saw it shoot three or
four Degrees farther, but with a Light so very faint that I could scarce
perceive it, and suspected it might (in some measure at least) arise
from some other cause than the two streams did. For placing my Eye in
185