VOL. XXIX.] PHILOSOPHICAL TRANSACTIONS. ^^Q 



fig. 3. Then at the distance of Q feet, looking through the prism c, at the 

 paper thus coloured, the red half appeared very much separated from the purple, 

 the one seeming lifted up from the other; the red or the purple appearing the 

 highest, according as the refracting angle of the prism was either held upwards 

 or downwards. The phenomenon is much more distinct this way than any 

 other; for the paper not only seems divided into two, when it is coloured by a 

 red and a purple ray, but also by a red and blue, as in fig. 4, by a red and a 

 green ray, as in fig. 5, or indeed by any two colours that are different, how 

 near soever their places in the spectra be to each other. The halves of the 

 paper appear, when viewed through the prism, to be farther from each other, 

 when the paper is tinged with such colours as are farther from eacli other in the 

 series of colours in the spectrum; and nearest, though still divided, when 

 neighbouring colours fall on the paper, as yellow and green, or a light and a 

 deep green. But the paper appears no way divided, when coloured with the red 

 of the two spectra, as in fig. 6, if those reds are equally intense; and so of the 

 other colours. 



Exper, III. — I held a lens, of about 3 feet radius, at the distance of 6 feet 

 from the oblong paper, on which a red and purple ray falling, made il look half 

 red and half purple; and I projected the image of the said coloured paper at the 

 distance of about 6 feet on the other side of the lens, on a sheet of white 

 paper; where it was observable, that when the red half was distinctly painted 

 on the white paper, which was known by the edges of the image being regularly 

 terminated, then the blue half of the image was confused; but when the white 

 paper was brought about 2 inches nearer to the lens, the image of the blue half 

 became distinct, and that of the red half confused. 



I tried the experiment with a paper coloured half red and half blue, the red 

 with carmine, and the blue with smalt, making the candle to enlighten the 

 paper, the room being otherwise dark ; and the experiment succeeded in the 

 same manner. The experiment thus made is the same that Sir Isaac Newton 

 gives an account of, book ], part 1, theor. 1, of his Optics. Only it is to be 

 observed, that when the oblong paper is coloured with red and blue from the 

 prisms, the focal place, where the red part of the image is distinct, is more 

 distant from the place where the blue part of the image is distinct, than when 

 the paper is coloured with the painter's powders, and much more vivid. 



Fig. 7 shows the projection of the paper tinged with the rays ; and fig. 8, 

 the projection of it when painted : where a black thread is wrapped round the 

 red and the blue part, that the distinctness of the image of the thread may 

 show when the red or when the blue part of the image of the paper is most 

 distinct. 



VOL. VI. H H 



