VOL. VI.] PHILOSOPHICAL TRANSACTIONS. (585 



compounded, and to its composition are requisite all the aforesaid primary co- 

 lours, mixed in a due proportion. I have often with admiration beheld, that 

 all the colours of the prism being made to converge, and thereby to be again 

 mixed as they were in the light before it was incident upon the prism, repro- 

 duced light, intircly and perfectly white, and not at all sensibly differing from 

 a direct light of the sun, unless when the glasses, I used, were not sufficiently 

 clear ; for then they would a little incline it to their colour. 



8. Hence therefore it comes to pass, that whiteness is the usual colour of 

 light ; for, light is a confused aggregate of rays indued with all sorts of colours, 

 as they are promiscuously darted from the various parts of luminous bodies. 

 And of such a confused aggregate, as I said, is generated whiteness, if there 

 be a due proportion of the ingredients ; but if any one predominate, the light 

 must incline to that colour; as it happens in the blue flame of brimstone ; the 

 yellow flame of a candle ; and the various colours of the fixed stars. 



'g. These things considered, the manner how colours are produced by the 

 prism, is evident. For, of the rays constituting the incident light, since those 

 which difler in colour, proportionally difter in refrangibility, they by their un- 

 equal refractions must be severed and dispersed into an oblong form in an or- 

 derly succession, from the least refracted scarlet, to the most refracted violet. 

 And for the same reason it is that objects, when looked upon through a prism, 

 appear coloured. For the difForm rays, by their unequal refractions, are made 

 to diverge towards several parts of the retina, and there express the images of 

 things coloured, as in the former case they did the sun's image upon a wall. 

 And by this inequality of refractions they become not only coloured, but also 

 very confused and indistinct. 



10. Why the colours of the rainbow appear in falling drops of rain, is also 

 from hence evident. For, those drops which refract the rays disposed to ap- 

 pear purple, in greatest quantity to the spectator's eye, refract the rays of other 

 sorts so much less, as to make them pass beside it ; and such are the drops on 

 the inside of the primary bow, and on the outside of the secondary or exterior 

 one. So those drops, which refract in greatest plenty the rays apt to appear 

 red, towards the spectator's eye, refract those of other sorts so much more, 

 as to make them pass beside it ; and such are the drops on the exterior part of 

 the primary, and interior part of the secondary bow. 



11. The odd phaenomena of an infusion of lignum nephriticum, leaf gold, 

 fragments of coloured glass, and some other transparently coloured bodies, ap- 

 pearing in one position of one colour, and of another in another, are on these 

 grounds no longer riddles. For, those are substances apt to reflect one sort of 

 light, and transmit another ; as may be seen in a dark room, by illuminating 



