1271 
Physics. — “The diffraction of Electromagnetic waves by a crystal.” 
3y Dr. L. S. Ornstetn. (Communicated by Prof. H. A. Lorentz). 
(Communicated in the meeting of Febr. 22, 1913). 
In the “Sitzungsberichte der Königl. Bayerischen Akademie der 
Wissenschaften” *) M. Lave has published a theory — and together 
with Messrs. FrieprricH and Knippinc experiments also — about this 
highly remarkable phenomenon. W. L. Brace, in a paper entitled 
“The diffraction of short electromagnetic waves by a crystal” *) 
doubts of the explication of this experiments given by Lave being 
satisfactory. He proposes an elementary theory, in which he points 
out that we can describe the phenomenon of Laue by regarding all 
as if the RÖNrGuN rays were reflected on the sets of planes that can 
be brought through the molecules of the crystal. In the following 
lines I will develop the theory proposed by Brage, and at the same 
time I will give a provisory discussion of some experiments made 
in the Physical Laboratory of the University of Groningen which 
Prof. Haga has been so kind as to put at my disposal, for which 
I may cordially thank him here. | 
I will confine myself to a regular crystal, the extension to crystals 
with other Bravats or SOHNKE point-systems being possible without 
any difficulty. 
1. Let us suppose a plane beam of RÖNtaeN rays (direction of 
ray: «-axis) to strike a regular crystal, of which one of the cubical 
axes of the point system is set parallel to the incident beam. The 
origin of coordinates is chosen in a molecule lying within the crystal 
in the middle of the part through which the rays are propagated, 
The y and z-axes are oriented parallel to the other cubic axis. Be the 
length of the side of the cubes a. The coordinates of a molecule 
of the crystal then are 
== hyo ym ka Zdenka EN 
in which #,, 4, and #, are positive or negative whole numbers. 
We shall examine the influence of the rays in a point with 
coordinates 5, 7, 5, at a distance r from the origin. 
Now whatever may be the constitution of primary RÖNTGEN rays, 
we can always imagine the disturbance of equilibrium being dissolved, 
according to the theorem of Fourter, into periodical movements. In 
1) Loc.cit. June 8 & July 6 1912. Interferenzerscheinunzen bei Röntgenstrahlen. 
2, Proc. Cambridge Phil. Soc. Vol. NX VIL, Part 1. The diffraction of short electro- 
magnetic w by a crystal. 
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