Colour which ought to arise by mixing those Rays (or their Colours) in
that proportion. I speak here of Colours so far as they arise from
Light. For they appear sometimes by other Causes, as when by the power
of Phantasy we see Colours in a Dream, or a Mad-man sees things before
him which are not there; or when we see Fire by striking the Eye, or see
Colours like the Eye of a Peacock's Feather, by pressing our Eyes in
either corner whilst we look the other way. Where these and such like
Causes interpose not, the Colour always answers to the sort or sorts of
the Rays whereof the Light consists, as I have constantly found in
whatever Ph nomena of Colours I have hitherto been able to examine. I
shall in the following Propositions give instances of this in the
Ph nomena of chiefest note.
_PROP._ VIII. PROB. III.
_By the discovered Properties of Light to explain the Colours made by
Prisms._
Let ABC [in _Fig._ 12.] represent a Prism refracting the Light of the
Sun, which comes into a dark Chamber through a hole F[Greek: ph] almost
as broad as the Prism, and let MN represent a white Paper on which the
refracted Light is cast, and suppose the most refrangible or deepest
violet-making Rays fall upon the Space P[Greek: p], the least
refrangible or deepest red-making Rays upon the Space T[Greek: t], the
middle sort between the indigo-making and blue-making Rays upon the
Space Q[Greek: ch], the middle sort of the green-making Rays upon the
Space R, the middle sort between the yellow-making and orange-making
Rays upon the Space S[Greek: s], and other intermediate sorts upon
intermediate Spaces. For so the Spaces upon which the several sorts
adequately fall will by reason of the different Refrangibility of those
sorts be one lower than another. Now if the Paper MN be so near the
Prism that the Spaces PT and [Greek: pt] do not interfere with one
another, the distance between them T[Greek: p] will be illuminated by
all the sorts of Rays in that proportion to one another which they have
at their very first coming out of the Prism, and consequently be white.
But the Spaces PT and [Greek: pt] on either hand, will not be
illuminated by them all, and therefore will appear coloured. And
particularly at P, where the outmost violet-making Rays fall alone, the
Colour must be the deepest violet. At Q where the violet-making and
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