Os Optici. 151

and nearer k: at /£they will be a pure green, but from thence towardsi they will incline gradually to a blue: at i they will be a perfect blue,inclining to an indigo colour from thence towards h: at h they willbe quite the colour of indigo, which will gradually change towards aviolet, the nearer they are to g: and at g they will be of a fine violetcolour,. which will incline gradually to a red as they come nearer to f twhere the coloured image ends.

There is not an equal quantity of rays in each of thefe colours; for,if the oblong image pq be divided into 360 equal parts, the red fpaceR will take up 45 of thefe parts ; the orange O, 27; the yellow T y48 ; the green G, 60 ; the blue B, 60; the indigo /, 40; and theviolet V, 80: ali which fpaces are as nearly proportioned in the figureas the fmall fpace qp would admit of.

If ali thefe colours be blended together again, they will make a purewhite; as isproved thus. Take away the paper on which the coloursp q feli, and place a large convex glafs D in the rays g , h, &c.which will refract them fo, as to make them unite and crofs each otherat W: and if a white paper be placed there to receive them, they willexcite the idea of a strong lively white. But if the paper be placedfarther from the glafs, as at r s, the different colours will appear againupon it, in an inverted order, occasioned by the rays Crossing at IV.

As white is a composition of all colours, fo black is a privatiori ofthem all, and, therefore, properiy no colour.

Let two concentric circles be drawn on a fmooth round board Fig. 6.ABCDEFG, and the outermost of them divided into 360 equalparts or degrees : then, draw feven right lines, as oA, O B, &c.from the center to the outermost circle, making the lines o A and © Binclude 80 degrees of that circle ; the lines o B and o C 40 degrees;o C and o D 60; o D and G E 60 ; o E and g F 48 j o F and©G 27; © G and 0^45. Then, between thefe two circles, paintthe fpace AG red, inclining to orange near G •, G F orange, incliningto yellow near F; FE yellow, inclining to green near E; E D green,inclining to blue near D-, D C blue, inclining to indigo near C , C Bindigo, inclining to violet near B ; and BA violet, inclining to a foftred near A. This done, paint all that part of the board black which Alt the prif.lies within the inner circle; and putting an axis through the centerof the board, let it be turned very fwiftly round that axis, so as the ge ther, malterays proceeding from the above colours, may be all blended and mixt a white.together in coming to the eye; and then, the whole coloured partwill appear like a white ring, a little greyifh; not perfectly white, be-caufe no colours prepared by art are perfect.

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