384 SCIENCE PROGRESS 



menting with crystals for which there is good ground to 

 suppose that the arrangement of the individual atoms differs 

 from that of the assemblages. 



To pass to the physical aspect of the phenomena, the fact that 

 the interference pattern is complete over a wide range of wave 

 lengths means that the incident radiation is analogous to white 

 light. It is not only the photograph reproduced here which 

 supports this idea but in other cases, when the photographs 

 which Laue obtained admit of analysis, the results conform to it. 

 The way in which the crystal builds up from the incident 

 radiation of all wave lengths the monochromatic trains of waves 

 which form the spots can perhaps be best understood by 

 considering the effect from a slightly different point of view. 

 Since the radiation from the bulb is to contain all wave lengths, 

 it may be regarded as a series of irregular pulses in the ether, 

 that is to say, considered as a whole and not split up into its 

 monochromatic components. It is in this way that Schuster 

 treats diffraction of white light by a line grating. Pulses of 

 this kind ought to undergo specular reflection at a plane surface 

 in just the same way that light or heat rays do, if the plane 

 surface differ in any way from the surrounding medium and if 

 its " polish" be sufficiently good. 



Taking advantage of the infinitely repeated pattern formed 

 by the atoms in a crystal, it is possible to classify them arbitrarily 

 as having their centres in sets of parallel planes. The simplest 

 of these planes are the cleavage planes of the crystal but, of 

 course, an infinite number of other ways are possible. When 

 the arrangement is made in a more complicated way, the planes 

 contain individually a very few atoms per unit area and are 

 crowded very close together ; on the other hand, the simple 

 cleavage planes are far apart and densely packed with atoms. 

 The " polish " of these planes is almost perfect ; at any rate the 

 irregularities due to the atom centres lying off the planes are 

 small compared with atomic dimensions (io~ 8 cm.) and there- 

 fore these planes will be capable of reflecting waves of wave 

 lengths io -9 cm. The spots in the interference pattern are 

 formed by the reflection of the incident beam in these planes in 

 the crystal. 



When a single pulse is reflected in one of these sets of 

 parallel planes the atoms in any one plane only scatter a fraction 

 of the energy in the pulse. The wavelets from all the atoms in 



