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.