On the Cutting of Polarising Prisms 1

Abstract

THE author showed the manner of cutting two new polarising prisms, designed by Ahrens and by himself, and described and figured in the Phil Mag. for June 1886. The Ahrens polariser is a rectangular parallelopipedon of calc-spar having square end-faces, and having its long sides in the proportion of about 1.6: 1 relatively to the short sides. The square end-faces are principal planes of section of the crystal. Two oblique sections are cut in the prism, being carried through the top and bottom edges of one end-face, and meeting in the horizontal middle line of the others. The dihedral angle between these planes of section is about 32°. The faces are polished and reunited with Canada balsam in the usual way. The advantages claimed for the new prism are: (1) decrease in length, (2) increase in angular aperture, (3) saving of light consequent on non-obliquity of end-faces, (4) minimum of distortion, (5) less spar required than in Hartnack, Glan, or Thompson prisms of same section. Against this are the slight disadvantages of (1) the line of section across end-face, and (2) the use of more spar than a Nicol of equal section. But Mr. Ahrens has recently added a thin covering-glass at the end-face crossed by the line of section, thereby making this line almost imperceptible; and he has also succeeded in finding a new method of cutting the prism in which there is extremely little waste of spar. The other prism designed by the author is a simple modification of the Nicol, giving a wider angle of field. A wedge is cut off each end of the calc crystal so as to make the new end-faces almost co-planar with a principal plane of section, and the crystal is cut through along the other diagonal of the sides. The results may be tabulated thus:—