es at a less distance, and
is more bent than when the Paper is held at a greater distance from the
Knives.

[Illustration: FIG. 3.]

_Obs._ 10. When the Fringes of the Shadows of the Knives fell
perpendicularly upon a Paper at a great distance from the Knives, they
were in the form of Hyperbola's, and their Dimensions were as follows.
Let CA, CB [in _Fig._ 3.] represent Lines drawn upon the Paper parallel
to the edges of the Knives, and between which all the Light would fall,
if it passed between the edges of the Knives without inflexion; DE a
Right Line drawn through C making the Angles ACD, BCE, equal to one
another, and terminating all the Light which falls upon the Paper from
the point where the edges of the Knives meet; _eis_, _fkt_, and _glv_,
three hyperbolical Lines representing the Terminus of the Shadow of one
of the Knives, the dark Line between the first and second Fringes of
that Shadow, and the dark Line between the second and third Fringes of
the same Shadow; _xip_, _ykq_, and _zlr_, three other hyperbolical Lines
representing the Terminus of the Shadow of the other Knife, the dark
Line between the first and second Fringes of that Shadow, and the dark
line between the second and third Fringes of the same Shadow. And
conceive that these three Hyperbola's are like and equal to the former
three, and cross them in the points _i_, _k_, and _l_, and that the
Shadows of the Knives are terminated and distinguish'd from the first
luminous Fringes by the lines _eis_ and _xip_, until the meeting and
crossing of the Fringes, and then those lines cross the Fringes in the
form of dark lines, terminating the first luminous Fringes within side,
and distinguishing them from another Light which begins to appear at
_i_, and illuminates all the triangular space _ip_DE_s_ comprehended by
these dark lines, and the right line DE. Of these Hyperbola's one
Asymptote is the line DE, and their other Asymptotes are parallel to the
lines CA and CB. Let _rv_ represent a line drawn any where upon the
Paper parallel to the Asymptote DE, and let this line cross the right
lines AC in _m_, and BC in _n_, and the six dark hyperbolical lines in
_p_, _q_, _r_; _s_, _t_, _v_; and by measuring the distances _ps_, _qt_,
_rv_, and thence collecting the lengths of the Ordinates _np_, _nq_,
_nr_ or _ms_, _mt_, _mv_, and doing this at several distances of the
line _rv_ from the Asymptote DD, you may find as many points of these
Hyperbola's as you