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 of the Knives measured from the point of their concourse to
the distance of the edges of the Knives at the end of that Length, and
so is the fifth Part of an Inch to the 160th Part. So then the dark
Lines above-mention'd meet in the middle of the Light which passes
between the Knives where they are distant the 160th Part of an Inch, and
the one half of that Light passes by the edge of one Knife at a distance
not greater than the 320th Part of an Inch, and falling upon the Paper
makes the Fringes of the Shadow of that Knife, and the other half passes
by the edge of the other Knife, at a distance not greater than the 320th
Part of an Inch, and falling upon the Paper makes the Fringes of the
Shadow of the other Knife. But if the Paper be held at a distance from
the Knives greater than the third Part of an Inch, the dark Lines
above-mention'd meet at a greater distance than the fifth Part of an
Inch from the end of the Light which passed between the Knives at the
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