A NEW THEORY OF LIGHT AND COLOURS. 467 



which they did before separation. And for the same reason, trans- 

 mutations made by the convening of divers colours are not real; for 

 when the difform rays are again severed, they will exhibit the very 

 same colours, which they did before they entered the composition; 

 as you see, blue and yellow powders, when finely mixed, appear to 

 the naked eye green, and yet the colours of the component corpuscles 

 are not thereby really transmuted, but only blended. For, when 

 viewed with a good microscope, they still appear blue and yellow 

 interspersedly. 



5. There are therefore two sorts of colours. The one original 

 and simple, the other compounded of these. The original or primary 

 colours are, red, yellow, green, blue, and a violet-purple, together with 

 orange, indigo, and an indefinite variety of intermediate gradations. 



6. The same colours in specie with these primary ones may be also 

 produced by composition: for a mixture of yellow and blue makes 

 green; of red and yellow makes orange; of orange and yellowish green 

 makes yellow. And in general, if any two colours be mixed, which 

 in the series of those, generated by the prism, are not too far distant 

 one from another, they by their mutual alloy compound that colour, 

 which in the said series appears in the midway between them. But 

 those which are situated at too great a distance, do not so. Orange 

 and indigo produce not the intermediate green, nor scarlet and green 

 the intermediate yellow. 



7. But the most surprising and wonderful composition was that 

 of whiteness. There is no one sort of rays which alone can exhibit this. 

 It is ever compounded, and to its composition are requisite all the 

 aforesaid primary colours, mixed in a due proportion. I have often 

 with admiration beheld, that all the colours of the prism being made 

 to converge, and thereby to be again mixed as they were in the light 

 before it was incident upon the prism, reproduced light, intirely and 

 perfectly white, and not at all sensibly differing from a direct light 

 of the sun, unless when the glasses, I used, were not sufficiently clear; 

 for then they would a little incline it to their colour. 



8. Hence therefore it comes to pass, that whiteness is the usual 

 colour of light; for, light is a confused aggregate of rays indued 

 with all sorts of colours, as they are promiscuously darted from the 

 various parts of luminous bodies. And of such a confused aggregate, 

 as I said, is generated whiteness, if there be a due proportion of the 

 ingredients ; but if any one predominate, the light must incline to that 

 colour ; as it happens in the blue flame of brimstone ; the yellow flame 

 of a candle ; and the various colours of the fixed stars. 



9. These things considered, the manner how colours are produced 

 by the prism, is evident. For, of the rays constituting the incident 

 light, since those which differ in colour, proportionally differ in 



