riangular Crystalline Prism, one of whose edges _P_.
is placed directly towards the Sun.

_A B_ & [alpha] [beta] Two rays from the Sun falling on the Prism at _B_
[beta]. and thence partly reflected towards _C_ & [gamma]. and partly
refracted towards _D_ & [delta].

_B C_ & [beta] [gamma]. Those reflected Rays.

_B D_ & [beta] [delta]. Those refracted Rays which are partly refracted
towards _E_ & [epsilon]. and there paint an Iris 1 2 3 4 5. denoting the
five consecutions of colours Red, Yellow, Green, Blew, and Purple; and are
partly reflected towards _F_ & [zeta].

_D F_ & [delta] [zeta]. Those Reflected Rays which are partly refracted
towards _G_ & [eta]. colourless, and partly reflected, towards _H_ &
[theta].

_F H_ & [zeta] [theta]. Those reflected Rays which are refracted towards
_I_ & [iota]. and there paint an other fainter Iris, the colours of which
are contrary to the former 5 4 3 2 1. signifying Purple, Blew, Green,
Yellow, Red, so that the Prism in this posture exhibits four Rainbows.

I know not whether you will think it Inconsiderable to annex to this
Experiment, That we observ'd in a Room not Darken'd, that the Prismatical
Iris (if I may so call it) might be Reflected without losing any of its
several _Colours_ (for we now consider not their _Order_) not onely from a
plain Looking-glass and from the calm Surface of Fair Water, but also from
a Concave Looking-glass; and that Refraction did as little Destroy those
Colours as Reflection. For by the help of a large (double Convex)
Burning-glass through which we Refracted the Suns Beams, we found that one
part of the Iris might be made to appear either beyond, or on this side of
the other Parts of the same Iris; but yet the same Vivid Colours would
appear in the Displac'd part (if I may so term it) as in the other. To
which I shall add, that having, by hiding the side of the Prism, obverted
to the Sun with an Opacous Body, wherein only one small hole was left for
the Light to pass through, reduc'd the Prismatical Iris (cast upon White
Paper) into a very narrow compass, and look'd upon it througn a Microscope;
the Colours appear'd the same as to kind that they did to the naked Eye.

_EXPERIMENT VI._

It may afford matter of Speculation to the Inquisitive, such as you,
_Prophilus_, that as the Colours of outward Objects brought into a Darken'd
Room, do so much depend for their Visibility upon the Dimness of the Light
they are there beheld by; that the o