514 



Dr. L. Vegard 



make the (001) spectrum normal, contrary to the obser- 

 vations, which give an abnormally weak second order 

 spectrum. 



We are then naturally led to assume the following 

 arrangement : 



Two oxygen atoms are associated with each Ti atom with 

 the latter in a central position forming a kind of molecular 

 element. In the case of anatase all the molecular axes are 

 parallel to the c-axis. 



The spacings corresponding to this arrangement are shown 



in fig. 4. 



Fig. 4. 

 Anarase 



7>- o 



! 

 I 



Ti 



(001) 



(100) l, 



r/o g 



(no) »; 



Ji0 2 

 (112) 



% 



TtO, 



TiO s 



O V 



We see that the faces (100) and (110) should give a 

 normal spectrum, and that the relative intensities of the 

 faces (001) and (112) should be equal. Both consequences 

 are in agreement with observations; it is only to be remarked 

 that in the case of the face (112) the reflexion is so weak 

 that the very weak spectra of second and third order could 

 not be detected at all. 



The arrangement has one parameter (the length of the 

 molecular axis), and it remains to be seen whether a proper 



