the Cause of coloured concentric Rings. 265 
action of bodies and surfaces on light would be better under- 
stood, if all the modifications wherein colours are produced 
had been before us, yet as the experiments I have to relate 
may be made plain, either by referring to modifications that 
are sufficiently known, or by explaining what is not already 
familiar, I shall postpone the intended enumeration to some 
future opportunity, and confine myself at present to a few re- 
marks relating to them. 
The colours contained in white light may be separated by 
reflection, as well as by refraction, and what is perfectly to 
my present purpose, the order, in which the colours thus pro- 
duced are arranged, is the same in both cases ; each of these 
principles therefore may cause coloured appearances, which 
the particular figure of the surfaces we use will mould into 
different configurations. 
Sir Isaac Newton, for instance, has shown that the rays of 
light will be separated, by what he calls a different reflexibi- 
lity, when they fall on the base of a prism ; the violet being 
reflected first, and the red last.* By this property of the dif- 
ferently coloured rays, he has explained a very remarkable 
phenomenon, which is that in a prism, when exposed in the 
open air, and when the eye is properly placed “ the spectator 
will see a bow of a blue colour/'-f- From the little the author 
has said of this bow, it may be supposed that he did not exa- 
mine it farther than was required for his purpose ; it will 
therefore be necessary to enter more fully into the subject. 
* See the illustration of the 9th experiment in the first book of Newton’s Optics, 
page 46. 
f See the 16th experiment in the second part of the first book, page 145. 
