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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1020. 



the composition of these planes. Imagine that the atoms of a cer- 

 tain crystal are so arranged that in a direction normal to some face 

 they lie in planes spaced as in figure 14. The distance apart of like 

 planes of these atoms is d; but midway between the planes made up 

 of atoms of A are planes of atoms of B. The waves reflected from 

 the A planes and from the B planes will interfere to an extent which 

 depends upon the relative reflecting powers of these atoms. If, for 

 example, the reflecting power of atom A were just equal to that of 

 atom B, it is then quite clear that the first order reflection from one 

 kind of plane, being of exactly opposite phase to that from the other, 

 would just nullify the reflection from this other plane so that the 

 distance apart of like planes would seem to be half of what it really 

 is. If the reflecting powers of the two atoms were not equal, then 

 there would still appear a first order reflection, but one weakened 



A B A B A B 



Fig. 14. 



to an extent dependent upon the difference in the scattering powers 

 of the two kinds of atoms. 



The " laws " of reflection. — The use of this kind of information in 

 deciding the arrangement of atoms in a compound necessitates that 

 the relative reflecting powers for X rays of the different atoms be 

 known. Previous studies of their scattering powers made it appear 

 that atoms scatter and reflect X rays in an amount very roughly 

 proportional to their atomic weights, or, more correctly, perhaps, 

 their atomic numbers. The reflections from various crystals seem 

 to have confirmed this rough proportionality ; but whether the " re- 

 flecting " power is strictly proportional to the number of electrons in 

 the atoms, and if not, what may be the exact nature of the function 

 expressing their scattering powers, is at present unknown. 



It has been stated that the use of the spectrometer makes possible 

 the estimation of the relative intensities of the reflections from vari- 

 ous crystal faces in the different orders. We have already seen how 

 the intensity of reflection can be taken as an indication of the com- 

 position of the reflecting planes. But in order to do this it is ob- 

 viously necessary to know the normal distribution of intensity in the 



