On the Phenomena of Binaural Audition. 351 



4. The author has succeeded in widening the available field 

 of polarized light by constructing polarizing prisms in which 

 this blue iris, and the total reflexion of the extraordinary ray 

 which produces it, are got rid of. This can be done by cut- 

 ting the crystal so that (1) the balsam film lies in a principal 

 plane of section, and (2) the crystallographic axis is at right 

 angles to the axis of the prism. 



The result of this mode of orientation of the axis and film 

 is to gain 9° of angular aperture at this side of the " field," 

 supposing the angles respectively made by the film and by the 

 terminal planes with the axis of the prism to be the same as in 

 the Mcol prism. 



It is possible to produce a further increase in width of avail- 

 able aperture at the other side of the field by reflecting back 

 the ordinary ray more than in the Nicol prism by making the 

 terminal faces more oblique ; but there is then more loss of 

 light by reflexion at the surfaces. 



5. Beside the advantage of a wider angular aperture, this 

 new form of polarizing prism has the advantage of producing 

 a field in which the rectilinear polarization approximates more 

 uniformly and symmetrically to a polarization in one plane 

 than is the case in the ordinary Nicol. There is, however, 

 more waste in cutting the spar, with proportionate increase in 

 cost. 



XLV. Phenomena of Binaural Audition. — Part III. 

 By Professor Silvanus P. Thompson, B.A., D.Sc* 



1. TN the author's papers of 1877 and 1878 he stated that, 

 J- when two separate simple tones reach the ears without 

 previously mingling, the difference-tone, or grave harmonic, 

 is not heard, though beats are if the tones are within beating- 

 distance. The latter statement referred to the beats of imper- 

 fectly-tuned unisons, which was the only case of primary 

 beats examined at the timef. 



* This paper, read before Section A of the British Association at York, 

 continues the researches communicated to the British Association by the 

 Author in 1877 and 1878, and printed in this Magazine. See p. 274* Oct. 

 1877, and p. 383, Nov. 1878. 



t This phenomenon of subjective interference, which was announced 

 in the author's paper of 1877, and further discussed in his paper of 1878, 

 was independently announced on Nov. 28, 1877, by Professor Graham 

 Bell as having been discovered by himself and Sir W. Thomson (subse- 

 quently to the publication of the author's first paper). The phenomenon 

 was also known in 1874 to Mach, who mentions it at the end of a paper 

 on the Functions of the Pinnae of the Ears (published in the Archtvfisr 

 Ohrenheilhunde), and ascribes it to a conduction of the sound through the 

 mass of the skull. 



