VOL. VII.] PHILOSOPHICAL TRANSACTIONS, 72Q 



and having a very small hole in it, be unequally refracted, since their inclina- 

 tion is unequal. Nor does it alter the case, that those rays are raised or de- 

 pressed by the rotation of the first prism, the second remaining immoveable, 

 (which however cannot be done in all cases) or contraryvvise the second being 

 turned while the first is fixed, that it may successively receive the coloured rays 

 of the whole image, and transmit them through its proper hole; for in either 

 case it is necessary that the extreme rays, viz. the red and the violet, should 

 fall on the second prism under unequal angles, and consequently that their 

 refraction be unequal, that of the violet being the greater. 



Since then here is an evident cause of that oblong figure of the rays, and 

 that cause such as arises from the very nature of refraction ; it seems needless to 

 have recourse to another hypothesis, or to admit of that divers refrangibility of 

 the rays. 



The author's notion of colours indeed follows very well from the preceding 

 hypothesis ; yet it is not without its difficulties. For when he says, that all the 

 rays being promiscuously blended together, yield no colour, but rather a white- 

 ness, this does not seem conformable to all the phaenomena. Doubtless the 

 same variations that are seen in the mixture of divers bodies of different colours, 

 are also observed in the mixture of different rays of various colours : and the 

 author himself has well observed, that as a green colour arises from a yellow and 

 a blue body, so likewise a green colour is produced from a yellow and a blue 

 ray. Therefore, if all the rays of the several colours be blended together, it is 

 necessary in that hypothesis, that that colour should appear, which in reality 

 arises on mixing together the several sorts of painters colours. That is,* as the 

 red, yellow, blue, purple, and all the others, when mixed together, produce, 

 not a white, but an obscure sated colour. So also ordinary light should appear 

 of the same colour, being a like aggregate of all the colours. 



Indeed nothing can be more ingenious and proper, than what he says about 

 Mr. Hook's experiment, in which are two different liquors, the one red, the 

 other blue, and each apart transparent, yet when mixed together they become 

 opaque : this the ingenious author thus explains: that the one liquor is disposed 

 to transmit only the red rays, the other only the yellow ; hence, both being 

 mixed together, they transmit none at all. But it should seem that the like 

 opacity should take place on the mixture of liquors of any other different colours ; 

 which however is far enough from the truth. 



VOL. I, 



4Z 



