the distances of the brightest parts of the Fringes which run between
_pq_ and _st_, _qr_ and _tv_, and next beyond _r_ and _v_, and found
them 0'5, 0'8, and 1'17 Inches.
_Obs._ 11. The Sun shining into my darken'd Room through a small round
hole made in a Plate of Lead with a slender Pin, as above; I placed at
the hole a Prism to refract the Light, and form on the opposite Wall the
Spectrum of Colours, described in the third Experiment of the first
Book. And then I found that the Shadows of all Bodies held in the
colour'd Light between the Prism and the Wall, were border'd with
Fringes of the Colour of that Light in which they were held. In the full
red Light they were totally red without any sensible blue or violet, and
in the deep blue Light they were totally blue without any sensible red
or yellow; and so in the green Light they were totally green, excepting
a little yellow and blue, which were mixed in the green Light of the
Prism. And comparing the Fringes made in the several colour'd Lights, I
found that those made in the red Light were largest, those made in the
violet were least, and those made in the green were of a middle bigness.
For the Fringes with which the Shadow of a Man's Hair were bordered,
being measured cross the Shadow at the distance of six Inches from the
Hair, the distance between the middle and most luminous part of the
first or innermost Fringe on one side of the Shadow, and that of the
like Fringe on the other side of the Shadow, was in the full red Light
1/37-1/4 of an Inch, and in the full violet 7/46. And the like distance
between the middle and most luminous parts of the second Fringes on
either side the Shadow was in the full red Light 1/22, and in the violet
1/27 of an Inch. And these distances of the Fringes held the same
proportion at all distances from the Hair without any sensible
variation.
So then the Rays which made these Fringes in the red Light passed by the
Hair at a greater distance than those did which made the like Fringes in
the violet; and therefore the Hair in causing these Fringes acted alike
upon the red Light or least refrangible Rays at a greater distance, and
upon the violet or most refrangible Rays at a less distance, and by
those actions disposed the red Light into Larger Fringes, and the violet
into smaller, and the Lights of intermediate Colours into Fringes of
intermediate bignesses without changing the Colour of any sort of Light.
When therefore the Hair in the first and second of these Observations
191