In a darkned Room, make a hole in the shut of a Window, whose Diameter may conveniently be about a third part of an Inch, to admit a convenient quantity of the Sun's Light. And there place a clear and colourless Prism, to refract the entring Light towards the farther part of the Room; which, as I said, will thereby be diffused into an oblong coloured Image. Then place a _Lens_ of 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 farther 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 _Prism_ and _Lens_ be placed steady, 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 entred 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, _Tab. 3. Fig. 2._ of this Experiment, _ABC_ expresseth the Prism set end-wise to sight, close by the hole _F_ of the Window _EG_. Its vertical Angle _ABC_ may conveniently be about 60 Degrees: _MN_ designeth the _Lens_. Its breadth 2½ or 3 Inches. _SF_ one of the streight Lines, in which difform Rays may be conceived to flow successively from the Sun. _FP_, and _FR_ two of those Rays unequally refracted, which the _Lens_ makes to converge towards _Q_, and after decussation to diverge again. And _HI_ 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 ρ, and _Blue_ and _Purple_ in _P_, _p_, and π.