892 Prof. W. H. Bragg on the Intensity of 



very obvious : and the spectrum of higher order is more 

 affected than that of lower order. As the crystal expands, 

 the spacing of the planes increases and the glancing angle 

 diminishes. This effect is also clearly seen. 



The table shows the variation of the effect with the mag- 

 nitude of the sine of the glancing angle. The latter is 

 expressed in terms of 6, the glancing angle of the first order 

 reflexion by the 100 plane : the value of sin 6 is 0*1097. 



Face. Glancing angle. I/I'. 



(100) 1 X sin 1-07 



(110) v/2 x „ 1-20 



(100) 2 x „ 1-26 



(110) 2^/2 x „ 2-07 



(100) 3 x „ 1-94 



The effects of temperature have been foreseen and calcu- 

 lated by Debije *, and the figures of the table may be looked 

 on as measurements of the Debije effect. 



It is interesting to compare the results given above with 

 the corresponding figures calculated from Debije's formula. 

 The latter may be written in the form 



where A and B are constants, given the crystal, the wave- 

 length, and the temperature, and 6 is the glancing angle 

 at which the pencil is reflected. I 6 is the intensity of the 

 reflected pencil. 



The constant B contains a quantity which Debije calls the 

 characteristic temperature of the crystal. Its value for 

 rocksalt is not known with certainty, but is believed to be 

 about 240°, In the following table the observed values are 

 set against calculated values. The principal reflexion from 

 the (100) face is always put equal to 100. 



The calculated values are slightly modified (become smaller) 

 if we take account of the possible existence of " nullpunkt 

 energie" and of an obliquity factor (1 + cos 2 20), where 6 is 

 the glancing angle. The two possibilities are considered by 

 Debije (loc. cit.). But the figures are hardly accurate enough 

 as yet to bear discussion in respect to these questions. . 



* Ann. der Phys. 1914, p. 49. 



