the Cause of coloured concentric Rings . 295 
It has been proved, that the different refrangibility of the 
prismatic colours, at certain critical angles, will cause the 
violet ; indigo, blue, and part of the green rays to be sepa- 
rately reflected, and that, according to what has been said in 
ths 49th article, this will produce an extended straight-lined 
appearance tinged with the abovementioned colours. It has 
also been shown that the same principle, at certain critical 
angles, will cause the red, orange, yellow, and part of the 
green rays to be exclusively intromitted, in such directions 
as will produce a similarly extended straight-lined appear- 
ance tinged v/ith these latter colours. From the angle in 
which the eye must receive these appearances in a prism, they 
are converted into the blue and red bows ; but, since they 
would appear to be straight lines, if they were seen in direc- 
tions perpendicular to a line drawn parallel to the edges of 
the prism, it follows, that were a long prism bent round into 
a circular form so that its two ends might meet, these lines 
would then be changed into rings, one of which would be 
formed by reflection, the other by transmission. 
A lens may be said to be such a prism, from which indeed 
it differs only in one respect, which is, that an angle contained 
between two lines applied as tangents to different parts of its 
surface is changeable, whereas the refracting angle of a given 
prism is constant. 
If it should be remarked that in consequence of considering 
a lens in this light, a plano-convex one, for instance, ought to 
present us, in certain situations, with a ring of the colours of 
the blue bow, and in others with a similar ring containing 
those of the red one, I must observe that the reason why such 
rings or bows can never be seen by the eye, though the phy- 
Q q 2 
