again upon the
middle of the second Board _de_, and there paint such an oblong coloured
Image of the Sun as was described in the third Experiment. By turning
the Prism ABC slowly to and fro about its Axis, this Image will be made
to move up and down the Board _de_, and by this means all its parts from
one end to the other may be made to pass successively through the hole
_g_ which is made in the middle of that Board. In the mean while another
Prism _abc_ is to be fixed next after that hole _g_, to refract the
trajected Light a second time. And these things being thus ordered, I
marked the places M and N of the opposite Wall upon which the refracted
Light fell, and found that whilst the two Boards and second Prism
remained unmoved, those places by turning the first Prism about its Axis
were changed perpetually. For when the lower part of the Light which
fell upon the second Board _de_ was cast through the hole _g_, it went
to a lower place M on the Wall and when the higher part of that Light
was cast through the same hole _g_, it went to a higher place N on the
Wall, and when any intermediate part of the Light was cast through that
hole, it went to some place on the Wall between M and N. The unchanged
Position of the holes in the Boards, made the Incidence of the Rays upon
the second Prism to be the same in all cases. And yet in that common
Incidence some of the Rays were more refracted, and others less. And
those were more refracted in this Prism, which by a greater Refraction
in the first Prism were more turned out of the way, and therefore for
their Constancy of being more refracted are deservedly called more
refrangible.

[Illustration: FIG. 18.]

[Illustration: FIG. 20.]

_Exper._ 7. At two holes made near one another in my Window-shut I
placed two Prisms, one at each, which might cast upon the opposite Wall
(after the manner of the third Experiment) two oblong coloured Images of
the Sun. And at a little distance from the Wall I placed a long slender
Paper with straight and parallel edges, and ordered the Prisms and Paper
so, that the red Colour of one Image might fall directly upon one half
of the Paper, and the violet Colour of the other Image upon the other
half of the same Paper; so that the Paper appeared of two Colours, red
and violet, much after the manner of the painted Paper in the first and
second Experiments. Then with a black Cloth I covered the Wall behind
the Paper, that no Light might be reflected