88 PHILOSOPHICAL TRANSACTIONS. [aNNO 1673. 



tween us, I shall endeavour to explain myself a little further in these things 

 according to the following method. 



Definitions. 



1 . I call that light homogeneal, similar or uniform, whose rays are equally 

 refrangible. 2. And that heterogeneal, whose rays are unequally refrangible. 



Note. There are but three affections of light in which I have observed its 

 rays to differ, viz. refrangibility, reflexibility, and colour; and those rays which 

 agree in refrangibility agree also in the other two, and therefore may well be 

 defined homogeneal, especially since men usually call those things homogeneal, 

 which are so in all qualities that come under their knowledge, though in other 

 qualities that their knowledge extends not to, there may possibly be some hete- 

 rogeneity. 



3. Those colours I call simple, or homogeneal, which are exhibited by homo- 

 geneal light. 4. And those compound or heterogeneal, which are exhibited by 

 heterogeneal light. 



5. Different colours I call not only the more eminent species, red, yellow, 

 green, blue, purple, but all other the minutest gradations; much after the same 

 manner that, not only the more eminent degrees in music, but all the least gra- 

 dations, are esteemed different sounds. 



Propositions. 



1 . The sun's light consists of rays differing by indefinite degrees of refrangi- 

 bility. 2. Rays which differ in refrangibility, when parted from one another, 

 do proportionally differ in the colours which they exhibit. These two proposi- 

 tions are matter of fact. 3. There are as many simple or homogeneal colours 

 as degrees of refrangibility. For to ever}^ degree of refrangibility belongs a 

 different colour, by prop. 2. And that colour is simple by def. 1 and 3. — 

 4. Whiteness, in all respects like that of the sun's immediate light, and of all 

 the usual objects of our senses, cannot be compounded of two simple colours. 

 For such a composition must be made by rays that have only two degrees of 

 refrangibility, by def. 1 and 3 ; and therefore it cannot be like that of the sun's 

 light, by prop. 1 ; Nor, for the same reason, like that of ordinary white 

 objects. 



5. Whiteness, in all respects like that of the sun's immediate light, cannot 

 be compounded of simple colours without an indefinite variety of them. For 

 to such a composition there are requisite rays indued with all the indefinite de- 

 grees of refrangibility, by prop. 1. And those infer as many simple colours, by 

 def. 1 and 3, and prop. 2 and 3. 



To make these a little plainer, I have added also the propositions that follow. 



6. The rays of light do not act on one another in passing through the same 



