on some New Polarizing Prisms. 477 



the usual faint red iris of interference-bands (fig. 2). The 

 breadth of the available polarized field at its narrowest part 

 is about 28°. Right and left there is no intrusion of ordinary 

 rays, the field extending to an angular breadth exceeding 

 100°. The blue iris, which in the ordinary Nicol prism is so 

 annoying at the limit of the field where the extraordinary ray 

 vanishes by total reflection, is entirely absent in this prism. 

 There is very little distortion, as might be expected from the 

 tetragonal form of the prism. 



The one disadvantage of the new prism is the presence of 

 the line of section S S' right across the face. This prevents 

 its being used, in some cases, as an analyzer, for which purpose 

 indeed other more suitable prisms exist, and for which this 

 prism was not designed. But it serves admirably as a polarizer, 

 the end A B C D, which is crossed by the section-line, being 

 turned towards the source of light. For use as a polarizer 

 with the optical lantern, or with the microscope, it is prefer- 

 able to the prisms of Nicol, Foucault, or Grlan, as it has a wider 

 angle than any of these, and does not lose light by oblique 

 reflection at its end-faces as the Nicol and Foucault do. Its 

 angle is not so wide as that of the prism described* by the 

 author of this note ; but it uses less spar than either that 

 prism or Hartnack's prism. Figs. 3 and 4 show Nicol and 

 Ahrens prisms of equal area of aperture-section. 



Since the limitation of the available field is due to the 

 intrusion of the ordinary ray, and does not depend on the 

 vanishing by total reflection of the extraordinary ray, there 

 is nothing to be gained by substituting in this prism any 

 cement such as linseed oil having a higher refractive index. 

 On the contrary, since the critical angle of total reflection for 

 ordinary rays increases as the index of refraction of the 

 cement-film decreases, it would be a gain to use, instead of 

 balsam, a cement of lower index. 



On suggesting to Mr. Ahrens that the prism might be cut 

 so that the section-line S S' bisected the face A B C D vertically 

 instead of horizontally (so making the section-planes principal 

 planes of section, as in the plan suggested by the writer for 

 polarizing prisms in 1881, at the York meeting of the British 

 Association) , Mr. Ahrens replied that he had already tried the 

 plan and that the result was the same. This should be so, 

 and should be so for any position of the line 8 S ; across the 

 face, provided the dihedral angle between the section-planes 

 is constant, because the effective field depends only on the 

 ordinary rays whose properties are symmetrical in all directions. 

 Nevertheless there is some advantage in making the films lie 

 * Phil. Mag. November 1881. 



