Chemistry and Physics. 455 



dots which were located with perfect symmetry with reference 

 to two mutually perpendicular axes. The dots on successive cir- 

 cles, however, alternated from one set of axes to a second set, so 

 that the entire figure presented an eight-fold symmetry. Dotted 

 figures were also obtained with crystals of copper sulphate, rock 

 salt, galena, and diamond. The greatest attention was given to 

 zinc blende because of its very simple crystal lographic structure. 

 In the case of the diamond the lateral photographic negatives 

 contained black dots as well as the emergence films. As yet this 

 apparent anomaly has not been explained. In all cases the figures 

 on the more remote emergence film were exact enlargements of 

 the figures on the film nearer the crystal, thus showing that the 

 secondary radiation is propagated in straight lines. (It may be 

 remarked that the necessary exposures lasted from 1 hour to 20 

 hours, and they were given intermittently to save the X-ray 

 bulbs.) 



The theory given in Laue's two papers agrees both qualita- 

 tively and quantitatively with the negatives obtained with zinc 

 blende. The crystal is considered as made up of elementary 

 cubes having molecules of sulphur and zinc at their vertices. 

 Consequently the entire crystal is equivalent to a perfect three- 

 dimensional diffraction grating. Also, the systems of photo- 

 graphic dots are analogous to the figures produced by crossed 

 gratings, in the corresponding two-dimensional problem. When 

 the primary X-rays pass through the crystal they cause the mole- 

 cules (or their constituents) to send out secondary (fluorescent) 

 X-rays which are in just the right condition for presenting inter- 

 ference phenomena, provided they consist in electromagnetic pulses 

 and not in corpuscles of some sort or other. The negatives show 

 the interference patterns and hence the conclusion must be drawn 

 that the secondary radiation is of the short-wave type. It is also 

 highly probable that the primary Rontgen rays are likewise ethereal 

 pulses, but further investigation will be required to fully establish 

 this hypothesis. The position of each of the dots on the negatives 

 fulfils the three necessary conditions for maxima of luminosity. 

 The loci of the directions in space which correspond to a differ- 

 ence in path from consecutive molecules of a whole number of 

 wave-lengths are circular cones. The cones whose axes are at 

 right angles to the axis of the primary pencil have very large 

 semi-vertical angles, and they intersect the emergence films in two 

 families of hyperbolas. The cones which are coaxial with the 

 primary pencil meet these films in circles, of course. When two 

 hyperbolas and a circle, all of the same order, meet in one and the 

 same point there will be a maximum effect and a dot will be 

 recorded on the film, if the exposure is adequate. It is necessary 

 but not sufficient, in general, for two plane loci to intersect. 



Assuming the number of molecules in a gram molecule or mol 

 to be 6*17 X 10 23 , and the molecular weight and density of zinc 

 sulphide as 97*4 and 4*06 gram/cm 3 respectively, the diameter of a 

 molecule calculated from Laue's formulae comes out to be 3-38 X 



