November 14, 1912] 



NATURE 



507 



ingly short wave length of the Rontg-en rays 

 (assuming them to be of electromagnetic wave 

 character), they had been able to penetrate the 

 crystal structure and to form an interference 

 {diffraction) photograph of the Bravais space- 



FlG. :. — Diagrammatic representation of Dr. Laue's apparati 



lattice. This latter is the structural foundation of 

 the more complicated regular point-system accord- 

 ing to which the crystal is homogeneously built 

 up, and the points of which (the point-system) 

 represent the chemical elementary atoms. The 

 space-lattice, in fact, was conceived to play the 

 same function with the short-wave Rontgen rays 

 that the diffraction grating does to the longer 

 electromagnetic waves of light. 



The details of this work were laid before the 

 Bavarian Academy of Sciences at Munich in two 

 memoirs, on June 8 and July 6 last, and the two 

 memoirs are now duly published in the Sitsungs- 

 berichte of the Academy.^ Besides a diagram of 

 the apparatus, which is reproduced in Fig. i, 

 they are illustrated by reproductions of a dozen 

 of these photographs, one of which is also repro- 

 duced in Fig. 2. There can be no doubt that 

 they are of supreme interest, and that they do 

 in reality afford a visual proof of the modern 

 theory of crystal structure built up by the 

 combined labours of Bravais, Sohncke, Schonflies, 

 von Fedorow, and Barlow. Moreover, they em- 

 phasise in a remarkable manner the importance 

 of the space-lattice, so strongly insisted on from 

 theoretical considerations by Bravais, Lord 

 Kelvin, and von Groth, and from experimental 

 considerations by Miers and the writer. They 

 further confirm the structure assigned to this 

 binary compound zinc sulphide, ZnS, by Pope 

 and Barlow. Incidentally they may form a 

 crucial test of the accuracy of the two rival 

 theories now being discussed as to the nature of 

 X-rays, the corpuscular and the wave theory. 



e>i Akad. der Wiss., Math. Phys. Kl., 



Out of an excellent crystal of zinc blende a plate 

 was cut a centimetre square and half a millimetre 

 thick, parallel to a cube face (loo), that is, per- 

 pendicular to one of the principal cubic crystallo- 

 graphic axes of the crystal (a tetragonal axis of 

 symmetry). The plate was supported in the usual 

 I manner on the crystal holder of a goniometer, 

 and precisely adjusted so that a beam of Rontgen 

 rays one millimetre in diameter impinged perpen- 

 dicularly upon it, after passing first through a 

 series of screens to eliminate secondary radiations 

 from the glass walls of the Rontgen tube. The 

 last screen, which gave the final form to the 

 bundle of rays, was a plate of lead a centimetre 

 thick, pierced by a cylindrical hole 0^75 millimetre 

 in diameter, and fitted with a de- 



~} licate means of adjustment so 



~R that the axis of the boring could 



be brought exactly perpendicular 

 to the crystal plate. The beam of pure Rontgen 

 rays of circular section thus passing through the 

 crystal normally was received, also normally, on a 

 Schleussner-Rontgen photographic plate, which 

 was subsequently developed with rodinal. The 

 time of exposure in different experiments varied 

 from one to tv^'enty hours, the whole apparatus 

 being excluded from all ordinary light by a 

 covering box. 



The positive print, reproduced in Fig. 2, 

 from the negative thus obtained shows a 

 central circular black spot, about half a centi- 

 metre in diameter, surrounded symmetrically 

 by sixteen smaller black spots of about the 

 same intensity, but of elliptical shape (about 

 two millimetres long), arranged in a diagonally 



NO. 2246, VOL. 90] 



I-'iC. 2.— Photogra,,hic effect of passage of ROntgen rays through zinc blende. 



(diamond-wise) placed square, four spots being 

 on each side of the square and separated from 

 each other by about half a centimetre, the centre 

 of the square being exactly occupied by the large 

 spot already alluded to, which was caused by the 



