ird Experiment, and then placed a second
Prism immediately after the first in a cross Position to it, that it
might again refract the beam of the Sun's Light which came to it through
the first Prism. In the first Prism this beam was refracted upwards, and
in the second sideways. And I found that by the Refraction of the second
Prism, the breadth of the Image was not increased, but its superior
part, which in the first Prism suffered the greater Refraction, and
appeared violet and blue, did again in the second Prism suffer a greater
Refraction than its inferior part, which appeared red and yellow, and
this without any Dilatation of the Image in breadth.

[Illustration: FIG. 14]

_Illustration._ Let S [_Fig._ 14, 15.] represent the Sun, F the hole in
the Window, ABC the first Prism, DH the second Prism, Y the round Image
of the Sun made by a direct beam of Light when the Prisms are taken
away, PT the oblong Image of the Sun made by that beam passing through
the first Prism alone, when the second Prism is taken away, and _pt_ the
Image made by the cross Refractions of both Prisms together. Now if the
Rays which tend towards the several Points of the round Image Y were
dilated and spread by the Refraction of the first Prism, so that they
should not any longer go in single Lines to single Points, but that
every Ray being split, shattered, and changed from a Linear Ray to a
Superficies of Rays diverging from the Point of Refraction, and lying in
the Plane of the Angles of Incidence and Refraction, they should go in
those Planes to so many Lines reaching almost from one end of the Image
PT to the other, and if that Image should thence become oblong: those
Rays and their several parts tending towards the several Points of the
Image PT ought to be again dilated and spread sideways by the transverse
Refraction of the second Prism, so as to compose a four square Image,
such as is represented at [Greek: pt]. For the better understanding of
which, let the Image PT be distinguished into five equal parts PQK,
KQRL, LRSM, MSVN, NVT. And by the same irregularity that the orbicular
Light Y is by the Refraction of the first Prism dilated and drawn out
into a long Image PT, the Light PQK which takes up a space of the same
length and breadth with the Light Y ought to be by the Refraction of the
second Prism dilated and drawn out into the long Image _[Greek: p]qkp_,
and the Light KQRL into the long Image _kqrl_, and the Lights LRSM,
MSVN, N