18 Dr. Brewster on the absorption of polarised light 
passes from O to C and D, its intensity diminishes gradually, 
the light becoming more and more white with a slight tinge 
of red, till it reaches C and D, where it has the same colour as 
that of the topaz in common light. As the plate of topaz is 
turned round the polarised ray, the blue tint changes into 
white, according to the law given in the preceding section, 
excepting in the line CD, where the tint is invariably white in 
every azimuth. When the polarised light is transmitted 
along any of the resultant axes P, P', the two tints diverge 
from the poles in the form of a cross, as shown in the pre- 
ceding figure. 
The effect at O, or the interchange of the blue and white 
light between the ordinary and extraordinary rays, is related 
to the axis AB alone ; and in like manner the effect at A is 
related solely to the other axis O. But though the axis O o 
is more powerful in topaz than AB, yet the values of m and n 
for each axis do not appear to be different ; a result which 
might have been expected from the fact, that these tints in 
different crystals have no relation to the intensity of their 
polarising forces. The diminution of the tints m and n, in 
passing from O to C and D, is owing to the action of the other 
axis O. At the points C and D blue light alone will be 
transmitted when AB, considered as a separate axis, is per- 
pendicular to the plane of primitive polarisation ; and at the 
same point white light alone will be transmitted when O con- 
sidered as a separate axis of the same character is similarly 
placed. Hence it follows, that the transmitted light should 
be bluish white, as it actually is, the interchanged portions 
being as it were in a state of equilibrium. 
The following table contains the tints m and n in several 
crystals, in which the effect will be seen in every azimuth. 
V 
