OF ELLIPTIC POLARIZATION. 
321 
in the plane +31° 52' the extraordinary ray will vanish, and the light will 
pass into the ordinary image; and if at 82° 30' we place it in —37° 22', the 
same effect will be produced. At 74° a small portion of light will pass into 
the extraordinary image, and this portion will gradually increase to 78° 8', the 
principal section of the prism having been turned round gradually from 
+31° 52' to 0°, as described in page 291. The ordinary and extraordinary 
images now approach most to equality, and they vary in intensity according 
to the same law in passing from 78° 8' to 82° 30', the axis of the prism having 
now come into the plane —37° 22'. The very same phenomena take place with 
red and blue light, only the points of restoration and the maximum occur at 
different angles of incidence, so that the spaces between the minima have dif- 
ferent lengths for the differently coloured rays. These spaces 
or loops, therefore, will overlap each other, as will be under- 
stood from the annexed diagram, where they are shown sepa- 
rately, r r' being the red loop, y y' the yellow, v v' the violet 
one, the points r, y, v, r', y\ v' the minima or nodes, and 
a, b, c the maxima. When these loops are viewed superposed 
as when they form white light, then the tint in the extraor- 
dinary image will be white, minus the three quantities of 
light that have disappeared from the extraordinary ray. At 
the line m n, passing through the node of the red loop, the 
red will have vanished, and the mixture of the yellow and the violet which 
remains will constitute a greenish blue pencil, decreasing in its blue tint 
towards a, and becoming pink, and then red towards t s, in consequence of 
part of the light of the other red loop above r now passing into the extraordi- 
nary ray. At v and at v\ where the violet disappears, the mixture of the yellow 
and the red will form an orange pencil, which will be reddest at v and v', and 
shading off to white at a. At the line s t the yellow vanishes, and across the 
upper part of the luminous disc, there will be light with an excess of red, and 
across the lower part of it, light with an excess of blue. This takes place with 
even numbers of reflexions ; with odd numbers the blue light is uppermost 
and the red undermost. 
The phenomena of colour, as seen by white light, vary greatly with the 
number of reflexions, both with respect to the depth of the colours themselves 
2 t 2 
