Undijracted Long-waved Heat Rays by Wire Gratings. 325 



On continuing the Fizeau-Ambronn investigation Prof. 

 Ferd. Brann* produced a partial Hertzian polarization o£ 

 light by means of certain structural appliances which were 

 produced by electric disintegration of metals, and, in par- 

 ticular, of platinum, as well as certain organic preparations — 

 wood and nettle fibres, &c. — " metallized " by means of gold 

 solution. He conjectured that he handled submicroscopic 

 grating structures, although this could not be directly proved; 

 he also assumed that grating polarization in the Hertzian 

 sense first appeared with distances smaller than half the 

 wave-length. 



§ 4. The effect on Hertzian waves of an infinitely long 

 metal cylinder was calculated by Dr. Seitz f for particular 

 cases, in which, however, d/X remained < 0-007. 



Dr. v. Ignatowsky J applied himself to the same problem, 

 and in addition, that of dielectric wires. 



Prof. Schaefer and Dr. Grossmann§ investigated the 

 diffraction of electromagnetic waves by dielectric (water-) 

 cylinders theoretically as well as experimentally. 



Prof. Schaefer and Dr. Reiche || have quite lately con- 

 tributed to the theory of grating inversion. The assumption 

 that the thickness of the wire is very small compared with 

 the wave-length, made in all the above theoretical contribu- 

 tions, is unavoidable in view of actual computation ; in fact 

 d/X ought in no case to be greater than unity, and it is better 

 for it to be less than one-half. 



Stress must be once again laid on the fact that the phe- 

 nomena here treated are only indirectly connected with 

 polarization caused by diffraction ; as we have repeatedly 

 remarked, the greater the wave-length the more do wire 

 gratings merge from diffracting appliances into pure pola- 

 rizers. The polarization of diffracted light by glass gratings 

 is comprehensively treated by FroblichU, by whom many 

 theoretical and experimental investigations of different ex- 

 perimenters, which are also of interest in connexion with 

 our investigations, are described. 



* Ferd. Braun, Ann. d. Phys. xvi. pp. 1 & 238 (1905) ; Physik. Zeitsckr. 

 v. p. 199 (1904;. See also O. Wiener, Physik. ZeiUchr. v. p. 332 Q904). 



f W. Seitz, Ann. d. Phys. xvi. p. 746 (1905), xix. p. 554 (1906). 



X W. v. Ignatowskv, Ann. d. Phys. xviii. p. 495 (1905). 



§ CI. Schaefer and F. Grosemann, Ann. d. Phys. xxxi. p. 455 (1910). 



|| 01. Schaefer and F. Reiche, Ann. d. Phys. xxxii. p. 577 (1910). 



^ J. Frohlich, Polar, of refracted light, Leipzig, 1907. See also 

 B. Pogany, Physik. Zeitschr. xii. p. 279 (1911). 



