*53 
the Cause of Coloured Concentric Rings. 
sudden change of the colours of the bows, which have been 
explained in my 43d and 45th articles, would still remain 
unaccounted for. 
Fourth consideration. The course of the rays which pro- 
duce the red bow, is so essentially different from the course 
of those which form the blue bow, that a mere inspection of 
the figures which represent them, proves, that one cannot be 
the converse of the other. The incident rays between A E, 
a. e, figure 4, are critically separated by the action of the 
interior base ; those which are reflected from the space be- 
tween a and e, go to H, and form the blue blow ; those which 
are transmitted through the same space, are by refraction 
scattered over an expansion of 9 0 1 3 contained between 
the mean refrangible green, passing from ce to g, and the least 
refrangible red, going from e to r. The incident rays in 
figure 5, which enter between A E and a. s, are likewise criti- 
cally separated, but the cause of this separation is the action 
of the exterior base of the prism ; and those which are intro- 
mitted between a and e so as to come to H, produce the red 
bow ; the rest being also intromitted, but not coming to H, 
are by refraction scattered over a space not exceeding 37' 7"; 
the most refrangible violet going from a to v, and the mean 
refrangible green, from e to g. As the smallness of some of 
the angles cannot be accurately expressed in the figures, they 
may be more correctly compared by the calculated particulars, 
which are as follows. 
Blue bow. Red bow. 
Convergency of the incident rays o° 2 1' 41 ",5 f 56' 50", 5 
Elevation of the vertices - - 49 57 3S 4 9 3 8 *9 >5 
Divergency of the scattered rays 9 11 4,3 o 37 7 
