22 PHILOSOPHICAL TRANSACTIONS. [aNNO 1672. 



rays indued with red, most difficultly the rays indued with violet, and with in- 

 termediate degrees of facility the rays indued with intermediate colours. So 

 that where the liquor is very thin, it may suffice to intercept most of the violet, 

 and yet transmit most of the other colours ; all which together must compound 

 a middle colour, that is a faint yellow. And where it is so much thicker, as 

 also to intercept most of the hlue and green, the remaining green, yellow, and 

 red, it must compound an orange. And where the thickness is so great, that 

 scarcely any rays can pass through it besides those indued w^ith red, it must ap- 

 pear of that colour, and that so much the deeper and obscurer, by how much the 

 liquor is thicker. And the same may be understood of the various degrees of 

 blue, exhibited by the solution of copper, by reason of its disposition to inter- 

 cept red most easily, and transmit a deep blue or indigo colour most freely. 



But supposing that all colours might, according to this experiment, be pro- 

 duced out of two by mixture ; yet it follows not, that those two are the only 

 original colours, and that for a double reason. First, because those two are 

 not themselves original colours, but compounded of others ; there being no 

 liquor nor any other body in nature, whose colour in day-light is wholly un- 

 compounded. And then, because, though those two were original, and all 

 others might be compounded of them, yet it follows not that they cannot be 

 otherwise produced. For I said that they had a double origin, the same colours 

 to sense being in some cases compounded, and in others uncompounded ; and 

 sufficiently declared in my third and fourth j^ropositions, and in the conclusion, 

 by what properties the one might be known and distinguished from the other. 

 But, because I suspect, by some circumstances, that the distinction might not 

 be rightly apprehended, I shall once more declare it, and further explain it by 

 examples. 



That colour is primary or original, which cannot by any art be changed, and 

 whose rays are alike refrangible : and that compounded, which is changeable 

 into other colours, and whose rays are not alike refrangible. For instance, to 

 know whether the colour of any green object be compounded or not, view it 

 through a prism, and if it appear confused, and the edges tinged with blue, 

 yellow, or any variety of other colours, then is that green compounded of such 

 colours as at its edges emerge out of it : but if it appear distinct, and well de- 

 fined, and entirely green to the very edges, without any other colours emerg- 

 ing, it is of an original and uncompounded green. In like manner, if a re- 

 fracted beam of light, being cast on a white wall, exhibit a green colour, to 

 know whether that be compounded, refract the beam with an interposed prism; 

 and if you find any diffi3rmity in the refractions, and the green be transformed 

 into blue, yellow, or any variety of other colours, you may conclude that it 



