TOL. VII.] PHILOSOPHICAL TRANS-ACTIONS. 21 



meter, and its length being greater or less accordingly as the refractions more 

 or less agreed, or contradicted one another. 



And by these observations, since the breadth of the image was not augmented 

 by the cross refraction of the second prism, that refraction must have been per- 

 formed without any splitting or diluting of the ray ; and therefore at least the 

 light incident on that prism, must be granted an aggregate of rays unequally re- 

 frangible in my sense. And since the image was equally inclined to both prisms, 

 and consequently the refractions alike in both, it argues that they were per- 

 formed according to some constant law without any irregularity. 



To determine the second quaere, the animadversor refers to an experiment, 

 made with two wedge-like boxes, recited in the Micrography of the ingenious 

 Mr. Hook, observation 10, page 73; the design of which was, to produce all 

 colours out of a mixture of two. But there is, I conceive, a double defect in 

 this instance. For it appears not, that by this experiment all colours can be 

 produced out of two ; and, if they could, yet the inference would not follow. 



That all colours cannot by that experiment be produced out of two, will ap- 

 pear by considering, that the tincture of aloes, which afforded one of those- 

 colours, was not all over of one uniform colour, but appeared yellow near the 

 edge of the box, and red at other places where it was thicker : affording all 

 variety of colours from a pale yellow to a deep red or scarlet,, according to the 

 various thickness of the liquor. And so the solution of copper, which afforded 

 the other colour, was of various blue and iixligoes. So that, instead of two 

 colours, here is a great variety made use of for the production of all others.. 

 Thus, for instance, to produce all sorts of greens,, the several degrees of yellow 

 and pale blue must be mixed; but to compound purples, the scarlet and deep 

 blue are to be the ingredients. 



Now if the animadversor contend, that all the reds and yellows of the one 

 liquor, or blue and indigoes of the other, are only various degrees and dilutings 

 of the same colour, and not divers colours, that is a begging of the question : 

 And I should as soon grant, that the two thirds or sixths in music are but 

 several degrees of the same sound, and not divers sounds. Certainly it is much 

 better to believe our senses, informing us, that red and yellow are divers colours, 

 and to make it a philosophical quasre, why the same liquor does, according to 

 its various thickness, appear of those divers colours, than to suppose them to 

 be the same colour because exhibited by the same liquor ? For, if that were a 

 sufficient reason, then blue and yellow must also be the same colour^ since they 

 are both exhibited by the same tincture of nephritic wood. But that they are 

 divers colours, you will more fully understand by the reason, which, in my 

 judgment is this : The tincture of aloes is qualified to transmit most easily the 



