VOL. VII.] PHILOSOPHICAL TRANSACTIONS. 25 



There are yet other ci renin stances by which the truth might have been de- 

 cided, as by viewing the white concourse of the colours through another prism 

 placed close to the eye, by whose refraction that whiteness may appear again 

 transformed into colours; and then, to examine their origin, if an assistant in- 

 tercept any of the colours at the lens before their arrival at the whiteness, the 

 same colours will vanish from amongst those, into which that whiteness is con- 

 verted by the second prism. Now if the rays which disappear be the same with 

 those that are intercepted, then it must be acknowledged, that the second 

 prism makes no new colours in any rays which were not in them before their 

 concourse at the paper. Which is a plain indication, that the rays of several 

 colours remain distinct from one another in the whiteness, and that from their 

 previous dispositions are derived the colours of the second prism. And, by 

 the way, what is said of their colours may be applied to their refrangibility. 



The aforesaid wheel may be also here made use of; and, if its gyration be 

 neither too quick nor too slow, the succession of the colours may be discerned 

 through the prism, whilst to the naked eye of a bystander they exhibit white- 

 ness. 



There is something still remaining to be said of this experiment. But this, 

 I conceive, is enough to enforce it, and so to decide the controversy. How- 

 ever, I shall now proceed to show some other ways of producing whiteness by 

 mixtures, since I persuade myself that this assertion, above the rest, appears 

 paradoxical, and is with most difficulty admitted. And because the animadver- 

 sor desires an instance of it in bodies of divers colours, I shall begin with that. 

 But in order thereto it must be considered, that such coloured bodies reflect 

 but some part of the light incident on them; as is evident by the 13th proposi- 

 tion; and therefore the light reflected from an aggregate of them, will be much 

 weakened by the loss of many rays. Whence a perfect and intense whiteness is 

 not to be expected, but rather a colour between those of light and shadow, or 

 such a grey or dirty colour as may be made by mixing white and black to- 

 gether. 



And that such a colour will result may be collected from the colour of dust 

 found in every corner of a house, which has been observ^ed to consist of many 

 coloured particles. There may be also produced the like dirty colour by mixing 

 several painters colours together. And the same may be eff^ected by painting 

 a top (such as boys play with) of divers colours. For, when it is made to cir- 

 culate by whipping it, it will appear of such a dirty colour. 



Now the compounding of these colours is proper to my purpose, because 

 they difl^er not from whiteness in the species of colour, but only in degree of 

 luminousness; which, did not the animadversor concede it, I might thus evince. 



VOL. II. E 



