The rest remaining as before, let P Q. (in fig. 127.) be the convex glass, causing the rays to meet upon the paper H G I K in the point N, I say that point or rather spot of light shall appear white, without the least tincture of any colour. But it is evident that into this spot are now gathered all those rays, which before when separate gave all those different colours; which shews that whiteness may be made by mixing those colours: especially if we consider, it can be proved that the glass P Q does not alter the colour of the rays which pass through it. Which is done thus: if the paper be made to approach the glass P Q, the colours will manifest themselves as far as the magnitude of the spectrum, which the paper receives, will permit. Suppose it in the situation _h g i k_, and that it then receive the spectrum _l m_, this spectrum shall be much smaller, than if the glass P Q were removed, and therefore the colours cannot be so much separated; but yet the extremity _m_ shall manifestly appear red, and the other extremity _l_ shall be blue; and these colours as well as the intermediate ones shall discover themselves more perfectly, the farther the paper is removed from N, that is, the larger the spectrum is: the same thing happens, if the paper be removed farther off from P Q than N. Suppose into the position θ γ η ϰ, the spectrum λ μ painted upon it shall again discover its colours, and that more distinctly, the farther the paper is removed, but only in an inverted order: for as before, when the paper was nearer the convex glass, than at N, the upper part of the image was blue, and the under red; now the upper part shall be red, and the under blue: because the rays cross at N.
12. NAY farther that the whiteness at the focus N, is made by the union of the colours may be proved without removing the paper out of the focus, by intercepting with any opake body part of the light near the glass; for if the under part, that is the red, or more properly the red-making rays, as they are styled by our author, are intercepted, the spot shall take a bluish hue; and if more of the inferior rays are cut off, so that neither the red-making nor orange-making rays, and if you please the yellow-making rays likewise, shall fall upon the spot; then shall the spot incline more and more to the remaining colours. In like manner if you cut off the upper part of the rays, that is the violet coloured or indigo-making rays, the spot shall turn reddish, and become, more so, the more of those opposite colours are intercepted.