•200 Professor William H.Bragg [June 5, 



critically as we have been able to study light, bv means of the spectro- 

 meter. " The wave-length of the X-ray has emerged as a measurable 

 quantity. The complete range of electromagnetic radiations now lies 

 before us. At one end are the long waves of wireless telegraphy, in 

 the middle are first the waves of the infra-red detected by their heat- 

 ing effects, then the light waves, and then the short waves of the 

 ultra-violet. At the other end are the extremely short waves that 

 belong to X-radiation. In the comparative study of the properties 

 of radiation over this very wide range we must surely find the answer 

 to the greatest question of modern physics. 



So much for the general question. Let us now consider the pro- 

 cedure of the new investigations, and afterwards one or two applica- 

 tions to special lines of enquiry. 



The experiment due to Lane and his collaborators Friedrich 

 and Knipping has ah'eady been described in this lecture room and is 

 now well-known. A fine pencil of X-rays passes through a thin 

 crystal slip and impresses itself on a photographic plate. Round the 

 central spot are found a large number of other spots, arranged in 

 a symmetrical fashion, their arrangement clearly depending on the 

 crystal structure. Laue had anticipated some such effect as the 

 result of diffraction by the atoms of the crystal. His mathematical 

 analysis is too complicated to be described now, and indeed it is not 

 in any circumstances easy to handle. It will be better to pass on at 

 once to a very simple method of apprehending the effect which was 

 put forward soon after the publication of Lane's first results. I 

 must run the risk of seeming to be partial if I point out the import- 

 ance of this advance, which was made by my son W. Lawrence Bragg. 

 All the recent investigations of X-ray spectra and the examination of 

 crystal structure and of molecular motions which have been carried 

 out since then have been rendered possible by the easy grasp of the 

 subject which resulted from the simpler conception. 



Let us imagine that a succession of waves constituting X-radiation 

 falls upon a plane containing atoms, and that each atom is the cause 

 of a secondary wavelet. In a well-known manner, the secondary 

 wavelets link themselves together and form a reflected wave. Just so 

 a sound wave may be reflected by a row of palings, and very short 

 sound waves by the fibres of a sheet of muslin. 



Suppose a second plane of atoms to lie behind the first and to be 

 parallel to it. The primary wave, weakened somewhat by passing 

 through the first plane, is again partially reflected by the second. 

 When the two reflected pencils join it will be of great importance 

 whether they fit crest to crest and hollow to hollow, or whether they 

 tend to destroy each other's effect. If more reflecting planes are 

 supposed, the importance of a good fit becomes greater and greater. 

 If the number is very large, then, as happens in many parallel cases in 

 optics, the reflected waves practically annul each other unless the 

 fit is perfect. 



