POLARIZED SPECTRA OF DOUBLE CHLORIDES. 



103 



The crystal C was mounted before the slit and turned about the axis 

 of the collimator until the planes of vibration of the transmitted light 

 coincided with planes of transmission of the rhomb. 



For the study of fluorescence, the light from a carbon arc A, after 

 passage through the condensing lens L, the water-cell W, and a light- 

 filter F, was employed for excitation. The filter was opaque to light 

 of a wave-length greater than 0.45ju, so that the fluorescence appeared 

 on a black background. When absorption spectrographs were required 

 a pale-blue screen was substituted, and the carbon arc was replaced by 

 a 1,000- watt nitrogen-filled tungsten lamp. 



C r 



9 L 



A. 

 X 



FIG. 70. 



Many crystals were produced before any were found which gave 

 complete separation of the two polarized components. A mere inspec- 

 tion of the crystals was not a sufficient criterion; but when trans- 

 mitted light polarized parallel to one of the planes of vibration of the 

 crystal was used, the presence of only one of the two absorption spectra 

 was found to afford a very delicate test, both for the adjustment of the 

 apparatus and the homogeneity of the crystal. In accordance with the 

 usage adopted we shall call that component of the spectrum due to 

 vibrations in the more transparent direction of the crystal, the white 

 component, while the component at right angles to this will be designated 

 as the green component. The stronger fluorescence, as might be expected, 

 is that of the green component, since light polarized in that plane is 

 more strongly absorbed. 



The four double chlorides upon which observations were made crys- 

 tallize in triclinic plates. These were so mounted that the flat faces 

 were at right angles to the transmitted light. 



The flat faces of the potassium, ammonium, and rubidium uranyl- 

 chloride crystals correspond to the (c) crystallographic face, while the 

 flat face of caesium-chloride crystals corresponds to the (&) crystallo- 

 graphic face. The caesium chloride crystallizes in gypsum-like plates, 

 which were mounted with the longest (c) crystallographic axis vertical. 

 Since the plane of polarization of the white light is also vertical within 



