X-RAYS AND CRYSTALS 387 



particular is hardly visible in the symmetrical pattern but 

 becomes the most intense of all when the crystal is tilted, because 

 its wave length is now right in the maximum of the spectrum. 

 If one imagines the crystal slowly tilted, the spots will be 

 in motion on the photographic plate and the wave length 

 continually changing. If only certain wave lengths were 

 present in the incident radiation, spots would be disappearing 

 and appearing all over the plate but as a matter of fact it can 

 be seen from the photographs that the process is quite con- 

 tinuous and so all wave lengths are present. 



The ellipticity of the spots can be easily understood if the 

 pencils forming them are regarded as rays reflected in crystal 

 planes. It is a geometrical result of the fact that the incident 

 pencil is not strictly parallel but slightly conical. If a conical 

 bundle of rays be reflected in a slip of crystal, at right angles to 

 its axis, regarded as a pile of parallel plates, the rays will come 

 to an approximate line focus on the far side at a certain distance 

 from the crystal and the ellipticity of the spots is a result of this 

 tendency. 



It was pointed out to me by Mr. C. T. R. Wilson that if 

 the interference phenomena could be regarded thus, it might 

 be possible to get a strong reflection in crystals which have 

 some very decided cleavage plane, this plane being presumably 

 thickly packed with atoms. To test this, a narrow beam of 

 X-rays was obtained by stops exactly as in Laue's experiments 

 and allowed to fall on a mica plate set so that the incidence was 

 almost glancing. A photographic plate was placed so that it 

 would receive both the transmitted beam and the reflected beam 

 if there were one. 



It was found that in this way a well-marked reflected spot 

 appeared after a few minutes' exposure to quite a weak bulb, 

 whereas Friedrich and Knipping found it necessary to expose 

 the crystal during many hours to the most intense beam of 

 X-rays obtainable in order to get good results. The effect is 

 almost a surface effect, quite thin plates of mica sufficing to give 

 full reflection ; it is possible that in this case the reflected radia- 

 tion is less penetrating and of greater wave length than that 

 forming the interference pattern with zinc blende. The rays 

 are, however, little absorbed by aluminium and black paper. 

 The reflected beam will, as before, consist of monochromatic 

 radiations, the wave length depending on the angle of incidence 



