186 M. F. Eisenlohr on the Relation between the direction of 



we caunot thereby solve the problem of the blue colour. Beu- 

 daut suggests that it may be owing to the hydration of the phos- 

 phates, instancing the example of the white anhydrous sulphate 

 of copper becoming blue on taking up its water of crj'stallization. 

 This might account very well for the production of colour in 

 compounds which pass from the anhydrous to the hydrated con- 

 dition, but cannot apply to the phosphate, inasmuch as the white 

 phosphate precipitated from protosalts of iron is hydrated while 

 in the white condition ; and it has never been shown that there 

 is any change in the quantity of water in passing from the blue 

 moditication. Then there is the formation of a blue phosphate 

 in the crucible, under conditions which preclude the possibility 

 of water of hydration. The change maybe allotropic, or per- 

 haps like the change of colours which light produces in some 

 kinds of glass, which gradually assume a decided pinkish tint, 

 although at first free from colour. This tint is due to manga- 

 nese ; but how are we to account for the gradual development of 

 the colour ? 



XXIX. On the Relation between the direction of the Vibration of 

 Light and the Plane of Polarization, and on its dctei-mination 

 by means of Diffraction. By Friedrich Eisenlohr of Hei- 

 delberg"^. 



THE question whether the particles of pether in a ray of polar- 

 ized light vibrate perpendicularly to the plane of polarization, 

 or in that plane, has been lately revived and earnestly discussed. 

 Two distinguished observers, Stokes and Holtzmann, have endea- 

 voured to decide the point by help of the pheenomenon of diffrac- 

 tion ; but though the availability of this method cannot be denied, 

 yet the two experimenters in question have in fact arrived at 

 different conclusions. As Stokes has not communicated his own 

 experiments, and as he does not appear to doubt the accuracy of 

 those of Holtzmann, we shall direct our attention principally to 

 the latter. 



Holtzmann found that a ray of polarized light which passed 

 through a glass plate, and was diffracted at its lower surface 

 through a grating of lampblack, was diffracted in greater quan- 

 tity when the plane of polarization before diffraction was parallel 

 to the bars of the grating, than when perpendicular; or, gene- 

 rally, that the plane of polarization of the incident ray was in 

 the diffracted ray turned towards the direction of the slits of the 

 diffraction grating. According to the theoretical considerations 

 that had led both him and Stokes to these experiments, this 



* Translated from PoggcndorfF's Annakn, vol. civ. p. o.'ij- 



