152 Intelligence and Miscellaneous Articles. 



ON THE DOUBLE REFRACTION OF ELECTRICAL WAVES. 

 BY PROF. AUGUSTE KIGHI. 



In No. 2, 1895, of Wiedemann's Annalen is a memoir by Prof. 

 Mach (Phil, Mag, July 1895), in which are described experiments 

 showing that a plate of pine-wood with faces parallel to the fibres 

 behaves towards electrical radiation just as a double refracting 

 plate towards luminous radiations. This memoir is dated Nov. 

 1894. 



Now on May 27, 1894, I communicated to the E. Accademia 

 di Bologna a long paper in which are described experiments equi- 

 valent to those of Prof. Mach, and others of analogous import. In 

 my experiments I not only confirmed the double refraction pro- 

 duced by wood, and the varying absorption of electrical rays 

 according as they are parallel or perpendicular to the fibres, but I 

 also obtained elliptical and circular polarization, using for this 

 latter a wooden plate of such a thickness as to represent a quarter 

 of a wave-length. Thus, like Prof. Mach, I compared the be- 

 haviour of wood to that of tourmaline rather than to that of calc- 

 spar or of quartz. One experiment of Prof. Mach's I did not make, 

 namely, that in which it is proved that there is no double refrac- 

 tion if the wooden plate is cut perpendicularly to the fibres. I 

 did not think it necessary to make this experiment, the idea of 

 which did indeed occur to me, because from reasons of symmetry 

 the result seemed a priori evident. 



I am satisfied that Prof. Mach did not know of my memoir, or 

 at any rate not when I made my communication to the Annalen, 

 and I am glad that his experiments so completely confirm those I 

 had already "published. I may add that, like him, I do not believe 

 in the possibility of producing double refraction of electrical 

 radiations by means of a plate of Iceland spar, for the reasons I 

 have explained in my memoir. In any case I intend soon to 

 resume my researches in these directions, which I have been pre- 

 vented from doing by other work. — Nuovo Cimento [4] vol. i., April 

 1895. Communicated by the Author. 



NOTE ON ELEMENTARY TEACHING CONCERNING FOCAL LENGTHS. 



To the Editors of the Philosophical Magazine. 

 Gentlemen, . 



In Dr. Barton's communication, page 59, I see he follows 

 the usual practice with regard to signs. My experience is that 

 students find it simpler and more natural to take the positive sign 

 for real images always, and the negative sign for virtual ones. 

 Similarly, converging instruments are to have their focal length 

 or their "potency" considered positive, and diverging instruments 

 negative, whether they be mirrors or lenses. Does Dr. Barton see 

 objection to this course ? 



Tours faithfully, 

 January 3, 1896. Oliver J. Lodge. 



