Knife, and three on the other side made by the edge of the other Knife.
They were distinctest when the Knives were placed at the greatest
distance from the hole in the Window, and still became more distinct by
making the hole less, insomuch that I could sometimes see a faint
lineament of a fourth Fringe beyond the three above mention'd. And as
the Knives continually approach'd one another, the Fringes grew
distincter and larger, until they vanish'd. The outmost Fringe vanish'd
first, and the middlemost next, and the innermost last. And after they
were all vanish'd, and the line of Light which was in the middle between
them was grown very broad, enlarging it self on both sides into the
streams of Light described in the fifth Observation, the above-mention'd
Shadow began to appear in the middle of this line, and divide it along
the middle into two lines of Light, and increased until the whole Light
vanish'd. This enlargement of the Fringes was so great that the Rays
which go to the innermost Fringe seem'd to be bent above twenty times
more when this Fringe was ready to vanish, than when one of the Knives
was taken away.
And from this and the former Observation compared, I gather, that the
Light of the first Fringe passed by the edge of the Knife at a distance
greater than the 800th part of an Inch, and the Light of the second
Fringe passed by the edge of the Knife at a greater distance than the
Light of the first Fringe did, and that of the third at a greater
distance than that of the second, and that of the streams of Light
described in the fifth and sixth Observations passed by the edges of the
Knives at less distances than that of any of the Fringes.
_Obs._ 8. I caused the edges of two Knives to be ground truly strait,
and pricking their points into a Board so that their edges might look
towards one another, and meeting near their points contain a rectilinear
Angle, I fasten'd their Handles together with Pitch to make this Angle
invariable. The distance of the edges of the Knives from one another at
the distance of four Inches from the angular Point, where the edges of
the Knives met, was the eighth part of an Inch; and therefore the Angle
contain'd by the edges was about one Degree 54: The Knives thus fix'd
together I placed in a beam of the Sun's Light, let into my darken'd
Chamber through a Hole the 42d Part of an Inch wide, at the distance of
10 or 15 Feet from the Hole, and let the Light which passed between
their edges fall very obliquely upon a smooth white Ruler at the
distance of half an Inch, or an Inch from the Knives, and there saw the
187