271 
the Cause of colowed concentric Rings . 
reflected, and passing through the opposite side of the prism 
projected the blue bow upon the deling of the room. The 
colours may there be conveniently seen ; but as this bow is 
composed of the least luminous rays of the prismatic spectrum, 
it requires considerable attention to perceive the faintest of 
them. The green and blue are most visible, and by receiving 
the bow upon a screen of white paper held at the most favour- 
able distance, the fainter colours, when the illumination is very 
bright may also be perceived. 
In order then to project also the red bow, I turned the prism 
upon its axis till the scattered light fell with a proper obliquity 
on the base of it ; the angle required for this purpose, it has 
been shown, must be from o° o' o" to 5 0 56' 50" 5 ; the side 
of the prism, which is turned towards the opening, should be 
covered with a slip of pasteboard to prevent any light from 
entering it. In this situation, I saw a very bright arch con- 
taining red, orange, and yellow projected at some distance 
backwards upon the deling ; that part of the green which no 
doubt was also transmitted, was lost in the brightness which 
is to be seen within the bow, for the same reason that the 
faint colours of the blue bow can only with great difficulty, if 
at all, be perceived ; namely, that they join the dark inside of 
the bow. For Newton has proved that the space beyond the 
convex part of the blue bow must be bright, and that beyond 
the concave dark ; but in the red bow, as my theory will 
show, we have that on the convex dark, and on the concave 
bright. This experiment therefore proves, that here, by the 
gradual intromission of the differently coloured rays, a criti- 
cal separation takes place on the outside of the prism, similar 
to that which by reflection happens in the blue bow at the in^ 
N n 2 
