Opticks 1717 - Up

fee Light which paGed by, &ll on a white Paper
two 01 three Feet beyond the Knife, and
there ftiw two @reams of faint Light fhoot out
both ways from the beam of Light into the &a-
HOW Iike the Tails of Comets. But becaufe the
Sun’s dirett Light by its brightnefs upon the
Paper d&zured thefe faint itreams, ib that’1
could fcarce fee themvi I made a ,little hole in
ihe midit of the Paper for that Light to pafs
through. 2nd fall on a. black Cloth behind it 5
pnd then1 faw the two itreams 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 dire&
Light was yrett itrong for the @ace of about
f quarter of an ry rich, or half an ‘Inch, and in all
Its progrefs from that dir&l Light deereaced
gradually till it became infenfible. The whole
length of’ either of thefe itreams meafured upon
the Paper at the diitance of three Feet from
the Knife was about fix or eight Inches ; fo that
it fubtended an Angle at the edge of the Knife
of about IO ot- 12, oi at mofi x.+ Degrees. Yet
fometimes I thought I faw it fhoot three or four
Degrees farther, but with aLight fo very faint
that I could Scarce perceive it, and fufpa%d it
might +( in fame meafure at leafi) arife from
fame other caufe than the,two Pcreams did. For
‘placing my Eye in that Light beyond the end
of that fiream which was behind the Knife, and
looking tokards the Knife, I could fee a line of
Light upon its edge, and that not only when
my Eye was i.n the line of the Streams; bu,t alfo
“‘when it was without that line either ‘tqwards
* the