the Cause of coloured concentric Rings . 273 
a lready been said in the 18th article of the first part of this 
paper of the change of the colours of rings. 
When we have before us a bow that is half blue and half 
red, it will be seen that both taken together contain all the 
prismatic colours in their regular order of refrangibility. It 
wi'l now also appear that the faint red, orange, and yellow, 
which I have said are to be perceived above the blue bow* 
may arise either from an imperfectly transmitted red bow, 
which always lies concealed under the Newtonian blue one, 
or perhaps more probably from the partial reflection of the 
red, orange, and yellow rays, many of which will come to 
the eye notwithstanding they are also copiously transmitted.^ 
According to my account of the red bow, it ought to be 
seen in the prism a little above the blue one, and this is also 
further confirmed by any one of the experiments in which we 
have some part of each bow in view at the same time, for then 
the relative situation of the two bows will be visible. 
Similar experiments may be made by candle light upon 
either of the bows ; for when a sheet of white paper is pinned 
against a wall, that it may reflect the light of a candle placed 
upon a table about three or four inches from the paper, we 
may then see the blue bow in a prism placed upon a dark 
ground before the reflecting paper ; and the green colour, 
which it is not very easy to perceive distinctly in daylight, will 
here be very visible, and the more so if we use an equilateral 
* See the first paragraph of the 41st article. 
f In my modifications of light I have proved, by undeniable experiments, that 
within a prism as well as on the outside of it the rays of all the colours are equally 
reflexible, and that a critical separation of them only takes place at those angles where 
by refraction a ray cannot be transmitted. 
