5<i 



NATURE 



[July 



19 i 3 



GREAT ADVANCE IN CRYSTALLOGRAPHY. 1 



TWO pictures of the actual apparatus employed 

 (one of which is produced in Fig. 7), and an 

 explanatory diagram of it (Fig. 8), will enable the 

 precise nature of the experiment to be grasped. A 

 plate, 1 cm. square and 05 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 bv 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 075 mm. in 

 diameter, and fitted with a delicate means of adjust- 

 ment so that the axis of the boring could be brought 



siderable number of other spots of elliptical shape, 

 arranged in a geometrical pattern. Three of these 

 original photographs are exhibited on the screen (and 

 two are also reproduced in Figs. 9 and 10). If a 

 series of such photographic plates be used, at different 

 distances from the crystal (as for Figs. 9 and 10), the 

 fact is revealed that the spots are formed by rectilinear 

 pencils of ravs spreading in all directions from the 



crystal, and some of them inclined more 



than 45 to the direction of the incident 



ravs. These deflected beams show 



similar properties to the original 



X-rays, ionising air and helium just 



like the latter," 



and with the 



same degree of 



variation with 



the pressure. > - a ^ • 



Hence, there 



can be no 



doubt that the 



character o f 



these deflected 



ravs issuing 



from the 



crystal is that 



of unaltered 



X-rays, and 



that" they are 



due to the 



deflection of 



X-ravs bv planes situated at ditierent 



angular positions in the interior of the 



crystal. In short, we are in face of 

 reflection of X-rays from planes of atoms in the 



Cr Nowa studv of the spots reveals the further interest 

 ing fact that' the pattern shows the full symmetrj 

 (that of class 32) of the cubic system to which the 





G. 9.— Spot photograph 

 afforded by zinc blende. 

 Incident X-rays perpen- 

 dicular to a cube-face, and 

 parallel to a tetragonal 

 axis of symmetry. 



Fig. 8 — Diagrammatic representation of apparatus of Friedrich, 

 Knipping, and Laue. A, Antikaihode of X-ray bulb ; B], B 2 , 

 I' 1. B_i. diaphragms of lead ; K. leaden box-screen with tubular 

 termination K ; S, large leaden screen ; G, goniometer ; 

 Pi. P=, Po. Pi. P51 photographic plates ; Kr, Cry ; tal ; Al. 

 aluminium plate. 



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 con- 

 Friday, March M . 



~ig. 10. — Spot photograph afforded by zin 

 pendicular to a cube-face, and parallel to 



ncider.t X-rays per- 

 axis of symmetry. 



1 From a discourse delivered at the Roval Institution or 

 by Dr. A. E. H. Tutton, F.R.S. (continued from p. 404). 



NO. 228l, VOL. 91] 



crystal belongs, although zinc blende exhibits the 

 slightly lower symmetry of the hexakis-tetrahedral 

 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 (same- 



