148 



angles being: 2736', 2330', and 20 respectively; and finally as 

 a spectrum of the third order with still smaller intensity, as B 3 and 

 C 3 , at angles of 3550' and 3048'. 



In agreement with the theory enunciated in the above, we find that: 

 sin 1348' : sin 2736' = 0,238: 0,463; sin 1130': sin 2330': sin 

 3550' - 0,199: 0,399: 0,585; and 



sin 10: sin 20: sin 3048' = 0,173: 0,342: 0,512; - - which ratios 

 are very near to 1 : 2, or to 1 : 2: 3. 



If instead of rock-salt, the corresponding mineral sylvine (KCl) 

 be used, the phenomena observed when reflection occurs at the 

 faces of the cube {100}, of the rhombicdodecahedron {110}, and 

 of the octahedron {111} successively, are in two of the three cases 

 wholly analogous in character, but for the same wave-length the 

 glancing angles on each of the three faces are different, their sines 

 being always in a constant ratio, exactly as in the case of rock-salt. 

 Thus for instance corresponding maxima on (100), 110), and (111) 

 are found here at 5 13', 7 18', and 93' respectively, the sines of 

 which are in proportion of 1 : 1/2 : 1/3. 



The same ratio wduld be found for the sines of the angles, at which 

 corresponding maxima occur on the faces (100), (110), and (111) 

 in the case of rock-salt, although the absolute values of these angles 

 are other than with sylvine. 



It is obvious that this constant ratio is exactly the same as that 

 of the inverse distances of the consecutive layers parallel to the 

 three faces mentioned in a simple cubic space-lattice. For if we take 

 the three possible types of arrangements in cubic space-lattices 

 (p. 118), we have: 



25. The behaviour of the sylvine-crysta.\ towards the R on t gen- 

 radiation can therefore easily be explained when the supposition 

 is made that the radiation observed is produced by particles arranged 



