four prisms, by whih the double refraction of the glass is directly indicated, 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 compression. 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 phænomena 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 compressing 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 circularly when that point of action corresponds to the points of division 45°, 135°, 225°, 315°, whilst 45° and 225°, as also 135° and 315°, exhibit 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
