152 



radiation of 1,486.10 8 cm., while a platinum-anticathode gives 

 a spectrum with five principal lines, the wave-lengths of the most 

 intense being: 1,316.10 8 cm. (A), a doublet of 1,113.10- 8 and 

 1,095.10 8 cm. (B), and a line of 0,96.10 8 cm. (C). 



Let us suppose that an X-ray-bulb is used with such a platinum 

 anticathode, of whose radiation we shall at present consider only 

 the wave-lengths denoted by A , B, and C. A crystal of sodiumchloride 

 may be so placed, that the "reflection" of the incident beam occurs 

 at the cube-face (100) of the crystal. 



Three maxima, of which B is the strongest and C the weakest, 

 are found at glancing angles $ of 1348 / , 1 130' and 10 respectively. 



They are repeated in a spectrum of the second order with somewhat 

 smaller intensities, as A 2 , B 2 and C 2 , the corresponding glancing 

 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 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 rocksalt, 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 rocksalt. 

 Thus, for instance, corresponding maxima on the faces (100), (1 10), 

 and (111) are found here at 513', 718 / , and 93' respectively, the 

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



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

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

 in the case of rocksalt, 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. 121), we have: 



