374 SCIENCE PROGRESS 



However, the experiments performed by Messrs. Friedrich, 

 Knipping and Laue have opened up a vast new range in the 

 spectrum never explored before. The paper in which they 

 announce their results appeared in June 191 2, in the Proceedings 

 of the Royal Bavarian Academy of Science. The effects which 

 they obtain can only be ascribed to waves of a length of the order 

 of one hundred millionth of a millimetre, the wave length being 

 small compared with the accepted radius of an atom! 



When dealing with visible light, a diffraction grating is most 

 commonly used in order to split up the mixture of light of all 

 colours into components the wave length of which can be 

 measured. The effect produced by the diffraction grating is a 

 consequence of the regularity of its structure ; it is ruled with 

 lines at constant intervals which are greater than the wave 

 lengths of the light to be examined but of the same order of 

 magnitude. It is the interaction of this regular spacing of the 

 lines and that of a train of waves composing monochromatic 

 light which leads to the appearance of interference maxima and 

 minima. Now there are reasons to suppose that X-rays consist 

 of electromagnetic waves of very short wave length, something 

 of the order io~ 9 cm. Laue came to the conclusion that if this 

 were so, it might be possible to get interference effects with these 

 short waves by using a crystal as a diffraction grating. The 

 atoms of a crystal are regularly arranged and on the whole the 

 intervals between them bear about the same relation to the wave 

 length io~ 9 cm. as does the " constant" of a diffraction grating to 

 the wave length of visible light. To these waves a crystal is 

 really a most perfectly ruled grating. The experiments to test 

 this prediction were carried out by Friedrich and Knipping at 

 Laue's request : they obtained a positive result with the first 

 crystal they tried. 



Since no way has yet been devised of obtaining a parallel 

 beam of X-rays corresponding to the parallel light which falls on 

 the grating in a spectroscope, an approximation to this must be 

 made. By a series of fine holes in screens of lead, the X-rays 

 from a bulb were stopped down till a very narrow pencil 

 1 mm. in diameter was obtained. 



This was allowed to fall on a small crystal of copper sulphate 

 and about 3 cm. from the crystal, in the direction away from the 

 bulb, a photographic plate was set perpendicular to the beam of 

 X-rays. The experimental arrangements are shown in fig. 1. 



