VOL. VII.] PHILOSOPHICAL TRANSACTIONS. 731 



seems to me, that the refractions of rays crossing each other, both in the an- 

 terior and posterior surface of the prism, may be justly calculated from my 

 principles. But if the case were otherwise, the breadth of the hole in the 

 posterior surface, if such there be, would hardly produce an error of two seconds; 

 and in practice such niceties may well be neglected. 



What the Rev. Father contends is not inconsistent with what I called the 

 Experimentum Crucis, viz. that the unequal refractions of rays endued with 

 different colours, were produced by unequal incidences : for transmitting rays 

 through two very small immoveable holes, and at a distance from each other, 

 the incidences, as I made the experiment, were always equal, and yet the re- 

 fractions were manifestly unequal. If he has any doubt of our experiment, I 

 request that he may measure the refractions of the said rays of divers colours 

 from equal incidences, and he will then see that they are unequal. But if he 

 dislikes the manner in which I have performed this matter (than which however 

 nothing can be clearer) it.is easy to devise other ways; as indeed I myself have 

 tried several other methods with advantage. 



Against the theory of colours it is objected, that powders of divers colours 

 mixed together do not yield a white, but an obscure and dusky colour. But 

 to me, white, black, and all the intermediate dusky colours, which cau be 

 compounded of mixtures of white and black, do not differ as to their species, 

 but only as to their quantity of light. And since in the mixture of painters 

 colours, each corpuscle reflects only its own proper colour, and therefore the 

 greatest part of the incident light is suppressed and retained; the reflected light 

 will become obscure, and as if mixed with darkness, so that it exhibits not an 

 intense whiteness, but an obscure dusky colour. 



Again it is objected that an opacity ought equally to arise from a mixture of 

 any liquors of different colours in the same vessel, as from the same liquors 

 contained in difi^erent vessels; which however he says is not true. But I see no 

 consequence in this. For many liquors act mutually on each other, and acquire 

 a new texture of parts; hence they may become opaque, or diaphanous, or of 

 various colours, in no manner owing to the colours of the compound. And 

 on that account I have always esteemed experiments of this kind not so proper 

 to draw conclusions from. It must also be noted that this experiment requires 

 liquors of full and intense colours, which transmit very few rays besides those 

 of their own colours; such as rarely occur, as will be seen by illuminating 

 liquors with difi^erent prismatic colours in a dark room. For few will be found 

 diaphanous enough in their own proper colours, and opaque in the others. Be- 

 sides, it is proper that the colours employed be opposites, such as I account red 

 and blue to be, or yellow and violet, or green and that purple which ap- 



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