82 DOVE’S EXPERIMENTS ON THE 
then will m — m,= (0 —e) ( . - = ): 
tox is a constant quantity for a definite substance, the dif- 
ference m—m, will be proportional to the quantity o—e. Hence it 
follows, That when for one definite colour the light is circularly pola- 
rized by an interposed crystallized lumina, it may for the other colours 
be linearly and oppositely circularly polarized, and that the difference 
between the single colours increases with the thickness of the lamina and 
with the intensity of the double refraction. 
If the incident light is cireular for the centre of the spectrum, when 
the difference of path is 1 for this centre, the light is not yet linear for 
a extreme limits of the spectrum. If it is here linear in the red, with 
4 undulation difference of path, in the blue it is circular. With 3 dif- 
feilbvas of path in the red, it will, if it is circular to the right, be linear 
in the blue, and circular to the left in the extreme violet. Linear light 
in the red, with difference of ‘path 1, gives on the left in the green a 
circular light, in the indigo a linear light perpendicular thereto and 
approaching the circular on the right in the extreme violet ; finally, on 
the left, circular in the red, with difference of path §, will give linear in 
the yellow, circular on the right where the blue passes into the indigo, 
and perpendicular to it linear at the commencement of the violet, and 
so forth. In order to prove this by experiment an equilateral prism of 
Guinand’s flint glass was placed upright, so that after the removal of 
the condensing-lens p the red end of the spectrum fell exactly upon the 
aperture e of the Nicol’s polarizing prism. The cube had by gradual 
heating exhibited the phenomena which corresponded to a difference 
of path of +, +, } undulation, and the other coloured rays were brought 
into the axis of the polarizing-apparatus, and the alteration of the Iceland 
spar figure examined. This might easily be accomplished without re- 
volving the prism, as the height of the instrument may be altered at 
pleasure by means of the sliding-tube, as may its inclination by means 
of the motion of the prismatic rod. Mica plates of various thicknesses 
were examined in the same manner as the heated cube. The changes 
may be seen most beautifully when, beginning with the violet, the instru- 
ment is slowly lowered in the sliding-tube through the single colours 
of the spectrum. The gradual transitions are, in respect to the difference 
of colours from one end of it to the other, exactly the same as those 
which are obtained by the heating and cooling of the cube. 
In the same manner the phenomena, when the incident light is cir- 
cularly analysed by a mica plate inserted before the Iceland spar, are 
throughout similar to those before described. Instead of the homo- 
geneous rays of the spectrum, we can of course also employ in these 
experiments a monochromatic lamp or absorption by coloured glasses. 
It is only when the light has been circularly polarized in one colour 
As 
