74 Mr. J. F. W. Herschel on the action of 
this examination, the ovals a, a , are drawn out to a surprising 
length, and the whole prismatic spectrum is displayed in them 
with great vividness of colour, while the violet portions of 
the rings are greatly elongated also, and appear to run into 
one another. If the plate be turned round in azimuth, the 
phenomena assume the most singular appearances of distor- 
tion ; and as the rotation approaches to 45 0 , the rings in the 
vicinity of the pole are gradually obliterated by their mutual 
overlapping, which is the greater the thicker the plate. In 
all situations, however, the interposition of a red or dark 
green glass immediately restores the perfect symmetry and 
regularity of the rings, which are then seen in much greater 
number, and completely well defined. 
All this is the necessary consequence of the want of coinci- 
dence of the axes for different colours. The lateral spots, 
for example, are formed for each homogeneous colour with 
perfect regularity close to their corresponding pole, and re- 
gularly decreasing in size from the red to the violet. Their 
arrangement will therefore be as represented in Fig. 3, Pl.V, 
R, O, Y, &c. being the poles corresponding to the several 
colours red, orange, &c. The oval spots composed of red 
rays being represented by r, r, those of the other colours 
will be super-imposed on them in their order, overlapping, as 
represented by the dotted ovals 00, yy, & c. like the circular 
coloured images of the sun in the spectrum of an ordinary 
prism, giving rise to the long prismatic tails above described. 
Similar considerations will apply to the anomalous appear- 
ances presented by the rings of all the other orders and in 
every situation. 
