CIRCULAR POLARIZATION OF LIGHT. 77 
four prisms, by which the double refraction of the glass is directly in- 
dicated, one of the two images which arise is polarized parallel to the 
axis of compression and the other perpendicular to it; whence it follows 
that the axis of the double refraction coincides with the axis of com- 
pression. If a square or circular plate of glass therefore is compressed 
so that the axis of compression forms an angle of 45° or 135° with the 
plane of primitive polarization, the light passing through the centre of 
the glass at a certain degree of the pressure will be circularly polarized. 
Let us now suppose a division of a circle so placed upon the incident 
ray that the plane of polarization passes through the points 90° and 270°; 
then, if the axis of compression passes through 45° and 225°, a plate of 
Iceland spar cut perpendicularly to the axis exhibits in the light passing 
through the centre of the compressed glass, instead of the black cross, 
rings in the second and fourth quadrants (on the right side above and 
on the left side below) advanced forwards by a quarter-interval from the 
centre, and on the contrary in the same proportion approaching nearer — 
to the centre when in the first and third quadrant (on the left above 
and on the right below). Exactly the reverse takes place when the 
axis of compression passes through the points of division 135° and 315°. 
Hence we see that the angles which in the parallelopiped of Fresnel 
are formed by the plane of the twice-repeated total internal reflexion 
with the plane of primitive polarization, must be equal to the angles 
under which the plane perpendicular to the axis of compression is 
- inclined towards the plane of primitive polarization, when the same 
; phenomena are to be produced by both those arrangements. 
No further particular explanation is now required to show that during 
a complete revolution of the plate in its plane round the perpendicular 
incident ray as an axis of revolution, the light is polarized four times 
rectilinearly and four times circularly; rectilinearly when the com- 
pressing screw acts on the points 0°, 90°, 180°, 270°, that is to say, 
when the axis of compression is perpendicular to the plane of primitive 
polarization or lies within it; and on the contrary, it is polarized cir- 
eularly when that point of action corresponds to the points of division 
45°, 135°, 225°, 315°, whilst 45° and 225°, as also 135° and 315°, ex- 
hibit a similar effect. 
By a combination of two compressed plates and two tourmaline plates, 
so that the mutually perpendicular axes of compression of the glass 
_ plates, which are between the crossed tourmaline plates, form with their 
axes an angle of 45°, a lamina of Iceland spar laid between the glass 
_ plates exhibits the rings without a cross with the black spot in the centre, 
and complementary ones on the contrary when we make the axes of the 
tourmalines or the axes of compression of the glass plates parallel to 
each other. If we make an axis of compression parallel to a tourmaline 
plate we obtain displacement of the rings in the four quadrants by a 
