Page:Philosophical Transactions - Volume 006.djvu/312

 about three foot radius (suppose a broad Object-glass of a three foot Telescope,) at the distance of about four or five foot from thence, through which all those colours may at once be transmitted, and made by its Refraction to convene at a further distance of about ten or twelve feet. If at that distance you intercept this light with a sheet of white paper, you will see the colours converted into whiteness again by being mingled. But it is requisite, that the Prisme and Lens be placed steddy, and that the paper, on which the colours are cast, be moved to and fro; for, by such motion, you will not only find, at what distance the whiteness is most perfect but also see, how the colours gradually convene, and vanish into whiteness, and afterwards having crossed one another in that place where they compound Whiteness, are again dissipated, and severed, and in an inverted order retain the same colours, which they had before they entered the composition, You may also see, that, if any of the Colours at the Lens be intercepted, the Whiteness will be changed into the other colours. And therefore, that the composition of whiteness be perfect, care must be taken, that none of the colours fall besides the Lens.



In the annexed design of this Experiment, A B C expresseth the Prism set endwise to sight, close by the hole F of the window E G. Its vertical Angle A C B may conveniently be about 60 degrees: M N designeth the Lens. Its breadth 2 or 3 inches. S F one of the streight lines, in which difform Rays may be conceived to flow successively from the Sun. F P, and F R two of those Rays unequally refracted, which the Lens makes to converge towards Q, and after decussation to diverge again. And H I the paper, at divers distances, on which the colours are projected: which in Q constitute Whiteness, but are Red and Yellow in R, r, and, and Blew and Purple in P, p, and.

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