94 



Dr. L. Vegard on 



intensities equal to 100, 20, 7, 3 for the orders 1, 2, 3, 4. 

 These numbers, however, contain the influence of tem- 

 perature, which will tend to diminish the intensities of 

 higher order as compared with those of low order. Cor- 

 rected for temperature effect the intensities should be 

 approximately inversely proportional to the square of the 

 order number. As also apparent from the way in which 

 Bragg has stated the law, it can only be considered as 

 tentative and as a first approximation ; and I want here to 

 give some facts which indicate that the intensity law cannot 

 be quite so simple. 



In the case of silver we have to deal with well-defined 

 crystals of only one element, and the two faces considered 

 (100) and (111) give both a normal spectrum*; but the 

 intensity falls off much more rapidly for the first than for 

 the second face. 



The face (111) gives an abnormally slow rate of fall with 

 increasing order. I have made a careful examination of 

 this point, comparing the first and second order of the two 

 faces, but could only confirm the result first obtained. Also 

 the (111) faces of gold and lead show the same abnormally 

 slow rate of fall. The results of the intensity measurements 

 are given in Table IX. 



Table IX. 



Face. 



Opening 

 of slit. 



Order. 



1. 



3. 

 7 



11 



Silver 



Gold. 

 Lead . 



(100) 

 (111) 



(111) 

 (111) 



0'5 min. 

 1 mm. 



05 mm. 

 0'5 mm. 



100 

 100 

 100 

 100 

 100 

 100 



100 

 100 



100 



20 



25 



195 



51 



50 



35 



35 



48 



49 



Also the face (110) of the Zircon group gives an 

 abnormally slow rate of fall of the intensities, even when 

 we take into account the effect of the oxygen atoms. Thus 

 in the case of tinstone, where the oxygen atoms can have 

 very little influence, the distribution corrected for the 



* Vesard, Phil. Ma<r. Jan. 1916. 



