6t)2 Dr. A. E. H. Tutton [March 14, 



precise nature of the experiment to be grasped. A plate, 1 cm. 

 square and 0*5 mm. thick, was cut from a good crystal of zinc 

 blende parallel to a cube face, and adjusted on the crystal holder of a 

 goniometer in the path of a very narrow pencil of X-rays from the 

 bulb, isolated by their passage through a succession of lead screens 

 (lead being impervious to X-rays) pierced by small holes. The last 

 screen, which gave the final form to the pencil of rays, was a plate of 

 lead 1 cm. thick, pierced by a cylindrical hole 0*75 mm. in diameter, 

 and fitted with a delicate means of adjustment so that the axis of the 

 boring could be brought exactly perpendicular to the crystal plate. 

 The beam of pure X-rays of circular section, after passing normally 

 through the crystal plate, was received on a Schleussner-Rontgen 

 photographic plate, which was afterwards developed with rodinal. 



The developed plate showed an intense circular spot at the centre, 

 caused by the direct X-rays, and a considerable -number of other 

 spots of elliptical shape, arranged in a geometrical pattern. Three 

 of these original photographs are exhibited on the screen (and 

 are also reproduced in Figs. 15, 16 and 17). If a series of such 

 photographic plates be used, at different distances from the crystal 

 (as in Figs. 15 and 16), the fact is revealed that the spots are 

 formed by rectilinear pencils of rays spreading in all directions from 

 the crystal, and some of them inclined over 45^ to the direction of 

 the incident rays. These deflected beams show similar properties to 

 the original X-rays, ionizing air and helium just like the latter, and 

 with the same degree of variation with the pressure. Hence, thei-e 

 can be no doubt that the character of these deflected rays issuing 

 from the crystal is that of unaltered X-rays, and that they are due to 

 the reflection of X-rays by planes situated at diff'erent angular posi- 

 tions in the interior of the crystal. In short, we are in face of 

 reflection of X-rays from planes of atoms in the crystal. 



Now a study of the spots reveals the further interesting fact that 

 the pattern shows the full symmetry (that of class 32) of the cubic 

 system to which the crystal belongs, although zinc blende exhibits 

 the slightly lower symmetry of the hexakis-tetraliedral class (31), one 

 of the formerly so-called hemihedral classes of the cubic system. 

 This clearly proves that 'it is the planes of similar and similarly 

 situated (sameways orientated) atoms in the crystal that are pro- 

 ducing the reflections, in other w^ords, the planes of the space-lattice. 

 At first Laue, who published a separate memoir on the theory of 

 the experiments, considered that it was the space-lattice due to 

 similarly situated zinc atoms which afforded the spot-pattern, a=; he 

 had been engaged with Prof. Summerfeld in experiments relating to 

 the action of zinc on X-rays. But there appears no reason why the 

 sulphur atoms should not be similarly capable of producing reflections 

 of these extremely fine vibrations or corpuscles, and as the space- 

 lattice is the same for both elements, according to all versions of the 

 geometrical theory of crystal structure, there is really no reason why 



