Experiment V.
PPP. An Aequilaterotriangular Crystalline Prism, one of whose edges P. is placed directly towards the Sun.
A B & α β Two rays from the Sun falling on the Prism at B β. and thence partly reflected towards C & γ. and partly refracted towards D & δ.
B C & β γ. Those reflected Rays.
B D & β δ. Those refracted Rays which are partly refracted towards E & ε. 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 & ζ.
D F & δ ζ. Those Reflected Rays which are partly refracted towards G & η. colourless, and partly reflected, towards H & θ.
F H & ζ θ. Those reflected Rays which are refracted towards I & ι. 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.