In order to isolate a polarized pencil of rays with a rhomb of Iceland spar, it is necessary to have a crystal of such a thickness that the emergent streams are separated, so that one may be stopped by a screen.
If this axis be parallel or perpendicular to the primitive plane of polarization, the emergent beam remains plane polarized; it is circularly polarized if the axis be at 45° to the plane of polarization, and in other cases it is elliptically polarized with the axes of the elliptic path parallel and perpendicular to the axis of the plate.
In general a stream of plane-polarized light undergoes no change in traversing a plate of an uniaxal crystal in the direction of its axis, and when the emergent stream is analysed, the light, if originally white, is found to be colourless and to be extinguished when the polarizer and analyser are crossed.
Hence one of the best methods of determining the magnifying power of a telescope to measure the diameter of the emergent pencil of rays, after the FIG.
Plane polarized light gives in general two streams of unequal intensity when examined with a rhomb, and for certain positions of the crystal there isonly one emergent stream.
On the other hand, if the emergent streams overlap and the common part be examined, it is found to have all the properties of common light.