506 Intelligence and Miscellaneous Articles. 



the wires being equal to their diameter ; by inclining the grating 

 relatively to the wave-front, the apparent clear spaces available for 

 the passage of radiation could be reduced to 1 mcin. The wave- 

 lengths investigated ranged from about | of the above unit down 

 to that of red light. The method employed was a somewhat 

 elaborate spectrobolometric one ; it was exclusively applied to the 

 non-diffracted part of the incident radiation, corresponding to the 

 so-called " central image/' 



It was found that all wire gratings polarize perpendicularly to 

 the direction of the wire so long as the wave-length is very small ; 

 for larger wave-lengths the action is, however, reversed. The 

 intermediate value of the wave-length, at which the grating does 

 not polarize, i. e. the " neutral point," is independent of the geo- 

 metrical constants of the grating, but characteristic for the metals 

 constituting its wires. Its value increases in the order Pt, Ag, 

 Au, Fe, Cu. The wire gratings behave like polarizers in every 

 respect. Malus' law, properly modified, was found to hold for the 

 polarizing action in different azimuths. The polarizing action in- 

 creases as the apparent clear space diminishes : the ratio of the 

 intensities of the two perpendicular components of radiation, 

 which measures that action, could be pushed up to over 2. The 

 results were checked by a simpler method of a more qualitative 

 character, which may prove useful for a repetition or demonstration 

 of the experiments. 



The authors then give a general discussion of the phenomenon, 

 its possible relations to other phenomena, the influence of diffrac- 

 tion, and, lastly, the possible explanation by it of the properties of 

 seolotropic dielectrics. This particular part of the paper presents 

 certain analogies with Lord Rayleigh's theoretical discussion of 

 " the influence of obstacles arranged in rectangular order upon the 

 properties of a medium " (Phil. Mag. Dec. 1892). 



However, the principal interest attaches to the experiments 

 described, when viewed in the light of the electric theory of radia- 

 tion and of Hertz's discoveries : the diminutive polarizing gratings 

 in the present case being quite analogous to the large electro- 

 magnetic wave-polarizers first introduced by that great investi- 

 gator. 



Since there can hardly remain a doubt that in the electromagnetic 

 theory, the electric vector must be considered perpendicular to the 

 plane of polarization, the magnetic vector parallel to it (Trouton, 

 Klemencic, Righi), the action of polarizing gratings in general 

 may now be described as follows : — 



As long as the wave-length keeps below a certain value charac- 

 teristic of the metal, the grating allows the greater fraction of 

 incident radiation to pass in case the electric vector be parallel to 

 the wires ; whereas for longer waves the transparency is greater 

 if the magnetic vector have that direction. For further particulars 

 the original may be referred to. — Wiedemann's Annalen, xlix. p. 593 

 (1893). 



