When, however, the planes of polarization and analysation are parallel or crossed, the two pairs of principal lines of like polarization coincide, and the intensity is at all points in the former case not greater than, and in the latter case not less than, it was before the introduction of the plate.
In order to correct this, the light after analysation is passed through another plate of quartz and then the sensitive tint may be more or less restored by cutting off some colour, the same for the whole field, by a Nicol's prism placed in the eyepiece of the telescope.
Poynting, is before analysation to impress unequal rotations upon the plane of polarization of the two parts of the field, either by means of an active medium, or by oblique transmission through glass plates.
According to Fresnel's explanation the light in each of the interfering streams consists of two trains of waves that are circularly polarized in opposite direction and have a relative retardation of phase, introduced by the passage through the quartz: the central fringes are then due to the similarly polarized waves; the lateral systems are produced by the oppositely polarized streams, these on analysation being capable of interfering.
If then a stream of polarized white light traverse the biquartz, it is possible by an analyser to cut off the mean yellow light from each half of the field, and the whole will then have the sensitive tint; but a small change in the plane of analysation will give the one half a red and the other half a blue tone.