370 M. G. Quincke on Diffraction. 



13. Furrow- and elevation-gratings of symmetrical form ex- 

 hibit so nearly the same behaviour towards polarized light, 

 if right and left are exchanged, that the phenomena may be re- 

 garded as identical. 



For the explanation of these phenomena, I think it must be 

 admitted that difference of phase and ratio of amplitude of the 

 light polarized parallel and perpendicular to the principal diffrac- 

 tion-plane depend on the diffraction- angle and also on the substance 

 and the magnitude of the boundary between the heterogeneous parts 

 of a grating upon which the unit of cross section of the incident 

 light falls. 



This tells in favour of an influence of the molecules of the 

 substance upon the oscillations of the sether particles, and the 

 inadmissibility of Huyghens's principle at the margins of the 

 apertures or furrows of a grating. 



The theorems found by experiment for gratings with groups 

 of apertures of like form, at equal distances from one another, 

 must hold also for gratings with similar-formed groups of aper- 

 tures at unequal distances from each other or for single groups 

 of apertures, and even for heterogeneous particles distributed 

 in a homogeneous base. Therein the distance and magnitude 

 of these particles may be less than a wave-length. 



Indeed polarized light exhibits the same behaviour towards 

 single slits and furrows, or towards gratings with like-formed 

 groups of apertures at unequal distances from one another, as, 

 according to the searching investigations of Fizeau*, it shows 

 towards ordinary gratings. Further, similar are the phenomena 

 of the polarization of the light of the sky which Aragof, Babi- 

 netj, and Brewster § have observed, the polarization pointed out 

 by Govi || and Tyndall^f in clouds of fine particles of dust and 

 vapour, and the polarization of diffused light which occurs when 

 diffraction is produced by very small heterogeneous particles dis- 

 tributed in water or other homogeneous transparent liquids or 

 solids, as described particularly by Soret** and Lallemandtt- 



All these experiments show that the light polarized parallel 

 to the plane of diffraction may have greater, less, or the same 

 intensity as that which is polarized perpendicular to the same 



* Comptes Rendusde V Acad, des Sci. vol. lii. pp. 267, 1221 (1861). 



t Werke, deutsch von Hankel, vol. vii. pp. 32/, 359 (1824). 



X Comptes Rendus, vol. xi. p. 619 (1840). 



§ Ibid. vol. xx. p. 802 (1845), xxiii. p. 234 (1846). 



|| Ibid. vol. li. pp. 360, 669 (1860). 



IT Phil. Trans. 18/0, p. 348. 



** Arch. d. Sc. Phys. vol. xxxv. p. 54 (1869), xxxvii. p. 148, xxxix 

 p. 1 (1870). 



ft Comptes Rendus, vol. Ixix. pp. 189, 282, 917, 1294 (1869), Ixx 

 p. 182 (1870), lxxv. p. 707 (1872). 



