684 1>HIL0S0PH1CAL TRANSACTIONS. [aNNO I67I. 



-nor by any other cause, that I could yet observe. When any one sort of rays 

 has been well parted from those of other kinds, it has afterwards obstinately re- 

 tained its colour, notwithstanding my utmost endeavours to change it. I have 

 refracted it with prisms, and reflected it with bodies, which in day-light were 

 of other colours ; I have intercepted it with the coloured "film of air interceding 

 two compressed plates of glass; transmitted it through coloured mediums, and 

 through mediums irradiated with other sorts of rays, and diversely terminated it; 

 and yet could never produce any new colour out of it. It would, by contract- 

 ing or dilating, become more brisk, or faint, and by the loss of many rays, in 

 some cases very obscure and dark; but I could never see it change in specie. 



4. Yet seeming transmutations of colours may be made, where there is any 

 mixture of divers sorts of rays. For in such mixtures, the component colours 

 appear not, but, by their mutual allaying each other, constitute a middling colour. 

 And therefore, if by refraction, or any other of the aforesaid causes, the difForra 

 rays, latent in such a mixture, be separated, there shall emerge colours differ- 

 ent from the colour of the composition. Which colours are not new generated, 

 but only made apparent by being parted; for if they be again entirely mixed and 

 blended together, they will again compose that colour, which they did before 

 separation. And for the same reason, transmutations 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 com- 

 position; 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 there^ 

 by really transmuted, but only blended. For, when viewed with a good micro- 

 "Bcope, 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 inter- 

 mediate yellow. 



7. But the most surprising and wonderful composition was that of white- 

 ness. There is no one sort of rays which alone can exhibit this. It is ever 



