238 Messrs. R. W. James and N. Tunstall on the 



small change in the spacing has a far greater effect on the 

 relative intensities in some positions than in others. In 

 the present experiments, the intensities of the orders were 

 measured roughly by the method of sweeping the crystal 

 round at a constant speed*, and for the first three orders are 

 probably known to within about 10 per cent. The fourth 

 order was too faint to be observed at all, and, although an 

 undoubted fifth order was found, its intensity cannot be con- 

 sidered as accurately known. The intensities of a series of 

 normal spectra were taken as 



100 :34 :14 : 7 : 4. 



These figures are based on comparisons of some measure- 

 ments on rock salt and on galena, and probably only represent 

 very roughly the normal intensities from antimony. The only 

 planes in this crystal which are evenly spaced are so close 

 together that the higher-order spectra occur at large angles, 

 and are therefore faint, so that no reliable intensity measure- 

 ments could be made for them. 



Assuming the normal intensities stated above, and calcu- 

 lating the intensities of three series of pairs of planes spaced 

 so that the phase-differences, 8, between the two sets for the 

 first-order spectra are 135°, 140°, and 145° respectively, we 

 get, taking the intensity of the second-order spectrum in each 

 case as 100, 



8 = 135° 86 : 100 : 71 : : 20 



8 = 140° 57 : 100: 51 : 1/4 : 18 



8 = 145° 39:100:38 : 3'5 : 17. 



The observed intensities were 



60:100:48 :0:15 



Supposing the intensities to have been measured to within 

 10 per cent., it will be seen that the value of 8 can hardly 

 vary by more than 2° on either side of 140° without causing 

 intensity changes greater than the errors of observation. 



Assuming now a different normal intensity law, supposing 

 the intensities to be inversely proportional to the squnre of 

 the order of the spectrum, which gives a ratio 



100 :25 : 3-1-1 :6'25 : 4, 

 * W. H. Bragg, Phil. Mag., May 1914, p. 881. 



