o 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
Fringes by the two edges of the Knives run along the edges of the
Shadows of the Knives in Lines parallel to those edges without growing
sensibly broader, till they met in Angles equal to the Angle contained
by the edges of the Knives, and where they met and joined they ended
without crossing one another. But if the Ruler was held at a much
greater distance from the Knives, the Fringes where they were farther
from the Place of their Meeting, were a little narrower, and became
something broader and broader as they approach'd nearer and nearer to
one another, and after they met they cross'd one another, and then
became much broader than before.

Whence I gather that the distances at which the Fringes pass by the
Knives are not increased nor alter'd by the approach of the Knives, but
the Angles in which the Rays are there bent are much increased by that
approach; and that the Knife which is nearest any Ray determines which
way the Ray shall be bent, and the other Knife increases the bent.

_Obs._ 9. When the Rays fell very obliquely upon the Ruler at the
distance of the third Part of an Inch from the Knives, the dark Line
between the first and second Fringe of the Shadow of one Knife, and the
dark Line between the first and second Fringe of the Shadow of the other
knife met with one another, at the distance of the fifth Part of an Inch
from the end of the Light which passed between the Knives at the
concourse of their edges. And therefore the distance of the edges of the
Knives at the meeting of these dark Lines was the 160th Part of an Inch.
For as four Inches to the eighth Part of an Inch, so is any Length of
the edges