Dkcembek 4j 1914] 



SCIENCE 



797 



orators Friedricli and Knipping has already 

 been described in this lecture room and is now 

 well-kno-wn. 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 ar- 

 rangement 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 in- 

 deed 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 Laue's first results. I must run 

 the risk of seeming to be partial if I point out 

 the importance of this advance, which was 

 made by my son W. Lawrence Bragg. AU the 

 recent investigations of X-ray spectra and the 

 examination of crystal structure and of molec- 

 ular 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 them- 

 selves 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 fibers 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 re- 

 flected 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 be- 

 comes 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. 



It is easily seen that the question of fit 

 depends on how much distance a wave reflected 

 at one plane loses in comparison with the 

 wave which was reflected at the preceding 

 plane : the fit wiU be perfect if the loss amounts 

 to one, two, three, or more wave-lengths ex- 

 actly. In its turn the distance lost depends 

 on the spacing of the planes, that is to say, 

 the distance from plane to plane, on the wave- 

 length and on the angle at which the rays 

 meet the set of planes. 



The question is formally not a new one. 

 Many years ago Lord Eayleigh discussed it in 

 this room, illustrating his point by aid of a 

 set of muslin sheets stretched on parallel 

 frames. The short sound waves of a high 

 pitched bird call were refiected from the set of 

 frames and affected a sensitive flame; and he 

 showed how the spacing of the planes must 

 be carefully adjusted to the proper value in 

 relation to the length of wave and the angle 

 of incidence. Eayleigh used the illustration 

 to explain the beautiful colors of chlorate of 

 potash crystals. He ascribed them to the re- 

 flection of light by a series of parallel and 

 regularly spaced twinning planes within the 

 crystal, the distance between successive planes 

 bearing roughly the same proportion to the 

 length of the reflected wave of light as the dis- 

 tance between the muslin sheets to the length 

 of the wave of sound. 



Our present phenomenon is exactly the same 

 thing on a minute scale: thousands of times 

 smaller than in the case of light, and many 

 millions of times smaller than in the case of 

 sound. 



By the kindness of Professor E. W. Wood I 

 am able to show you some flne examples of the 

 chlorate of potash crystals. If white light is 

 allowed to fall upon one of them, the whole 

 of it is not reflected. Only that part is re- 

 flected which has a definite wave-length or 

 something very near to it, and the reflected 

 ray is therefore highly colored. The wave- 

 length is defined by the relation already re- 

 ferred to. If the angle of incidence is altered, 

 the wave-length which can be reflected is 

 altered, and so the color changes. 



It is not difficult to see the analogy between 



