Fisr. 9. 



896 Prof. W- H. Bragg on the Intensity oj 



crystals experimented upon the ratio is more than 9 to 1. 

 The intensity in general falls off more rapidly than the 

 square of the sine of the glancing angle. 



For this reason I sought for some error in the assumptions. 

 It seemed possible that it might be a mistake to assume the 

 absorption coefficient to be the same when the rays met the 

 crystal at the proper angle for reflexion as when they did 

 not. The strong reflexion which is found experimentally 

 might take place at the expense of the transmitted ray. 

 If this were the case the crystal when reflecting should 

 absorb abnormally: it should be possible to find an absorption 

 band. R. W. Wood found such a band in the analogous case 

 of the reflexion of light by chlorate of potash crystals. 



Accordingly the pencil of rays AB was made to pass 

 through the diamond 0, which was 

 mounted on the revolving table of 

 the spectroscope, and alterwards 

 to meet a crystal of rocksalt at (7. 

 This was adjusted until it re- 

 flected into the ionization chamber 

 through the slit at D. The dia- 

 mond was then turned very 

 gradually through the angles 

 (8° 35' and 8° 39') at which it is 

 itself able to reflect. At the same 

 time the strengh of the pencil 

 reflected by the rocksalt was 

 continuously observed. It was 

 found that there was a con- 

 siderable decrease in the intensity 

 ot the pencil transmitted by the 

 diamond and reflected by the 

 rocksalt at exactly the angles 

 mentioned. Thus a special ab- 

 sorption does exist, and we under- 

 stand why the attempt to apply 

 a formula in which the ordinary 

 absorption coefficient is used leads 

 to unlikely results. Fig. 10 

 shows two curves : the upper 

 records the changes in the in- 

 tensity of the pencil reflected by 

 the rocksalt as the diamond is 

 turned, the lower the changes 

 in the case of the pencil reflected 

 by the diamond. The two sets 

 of measurements are made by 



