This is effected in polarizing prisms of the earlier type, devised by A.
Kempf with a polarizing photometer; but by far the most comprehensive work of the kind is the Harvard Photometric Durchmusterung (1901-1903), embracing all stars to 7.5 magnitude, and extended to the southern pole by measurements executed at Arequipa.
Examining the light reflected from the windows of the Luxemburg palace with a doubly refracting prism, he was led to infer (though more refined experiments have shown that this is not strictly the case) that light reflected at a certain angle, called the polarizing angle, from the surface of transparent substances has the same properties with respect to the plane of incidence as those of the ordinary stream in Iceland spar with respect to the principal plane of the crystal.
Further, if polarized light fall at the polarizing angle on a reflecting surface, the intensity of the reflected stream depends upon the azimuth of the plane of incidence, being proportional to the square of the cosine of the angle between this plane and the plane of the polarization.
At angles other than the polarizing angle common light gives a reflected stream .that behaves as a mixture of common light with light polarized in the plane of incidence, and is accordingly said to be partially polarized in that plane.