de_ and
_[Greek: de]_; the same Light in the one case appeared yellow and red,
in the other blue. Here one and the same part of the Light in one and
the same place, according to the various Inclinations of the Paper,
appeared in one case white, in another yellow or red, in a third blue,
whilst the Confine of Light and shadow, and the Refractions of the Prism
in all these cases remained the same.

[Illustration: FIG. 2.]

[Illustration: FIG. 3.]

_Exper._ 3. Such another Experiment may be more easily tried as follows.
Let a broad beam of the Sun's Light coming into a dark Chamber through a
hole in the Window-shut be refracted by a large Prism ABC, [in _Fig._
3.] whose refracting Angle C is more than 60 Degrees, and so soon as it
comes out of the Prism, let it fall upon the white Paper DE glewed upon
a stiff Plane; and this Light, when the Paper is perpendicular to it, as
'tis represented in DE, will appear perfectly white upon the Paper; but
when the Paper is very much inclin'd to it in such a manner as to keep
always parallel to the Axis of the Prism, the whiteness of the whole
Light upon the Paper will according to the inclination of the Paper this
way or that way, change either into yellow and red, as in the posture
_de_, or into blue and violet, as in the posture [Greek: de]. And if the
Light before it fall upon the Paper be twice refracted the same way by
two parallel Prisms, these Colours will become the more conspicuous.
Here all the middle parts of the broad beam of white Light which fell
upon the Paper, did without any Confine of Shadow to modify it, become
colour'd all over with one uniform Colour, the Colour being always the
same in the middle of the Paper as at the edges, and this Colour changed
according to the various Obliquity of the reflecting Paper, without any
change in the Refractions or Shadow, or in the Light which fell upon the
Paper. And therefore these Colours are to be derived from some other
Cause than the new Modifications of Light by Refractions and Shadows.

If it be asked, what then is their Cause? I answer, That the Paper in
the posture _de_, being more oblique to the more refrangible Rays than
to the less refrangible ones, is more strongly illuminated by the latter
than by the former, and therefore the less refrangible Rays are
predominant in the reflected Light. And where-ever they are predominant
in any Light, they tinge it with red or yellow, as may in some measure
appear by the fir