694 



Dr. A. E. H. Tutton 



[March 14, 



produced by a train of reflected pulses. The crystal thus actually 

 manufactures rays of definite wave-lengths, just as a diffraction 

 grating does, the only dift'erence being here in the extremely short 

 length of the waves, which is the very reason why X-rays can penetrate 

 in this manner into the Eaumgitter structure. Each incident pulse 

 produces a train of pulses, resolvable into a series of wave-lengths, 

 A, A/2, A/g, A/^, etc., where \ =^ 2 d cos 0, d being the shortest distance 

 between successive identical parallel planes in the crystal, and the 

 angle of incidence of the primary X-rays on the plane of points of 

 the space-lattice. The intensity of any spot depends on the energy 

 in the spectrum of the incident radiation characteristic of the 

 corresponding wave-length, and this varies considerably so that 

 certain parts of the spectrum are much more pronounced than others. 

 Also it depends on the number of reflecting atoms in the plane, that 

 is, on the reticular density of the possible crystal face corresponding 

 to the plane. Hence, the greater the reticular density, the more 

 intense the spot produced in the photograph. As reticular density is 

 also proportional to importance of face, the primary faces having the 

 greatest reticular density, it follows that the most important facial 

 planes reflect the intensest spots, a fact which may prove of great 



value in enabling us to 

 discover the real primary 

 planes in doubtful cases. 

 Each spot reflected by a 

 plane (considered as 

 passing through the 

 origin and two other 

 points) lies at the inter- 

 section of two ellipses, 

 and the figure on the 

 screen (reproduced in 

 Fig. 18), showing an 

 analysis of one of the 

 spot photographs, ex- 

 hibits this clearly. 

 Indeed, the plane of 

 atoms coiTesponding to 

 any spot can be found 

 from the two ellipses : 

 for each ellipse is the 

 section of a cone by the 

 plane of the photo- 

 graphic plate, the axis of the cone l)eing the line joining the origin 

 (centre of the triaxial system, and considered as one of the three 

 points determining the plane) and the particular atom (the second or 

 third point of the three, of definite co-ordinates), and the generator 

 of the cone beino; the incident beam. 



Pig. 18. — Analysis of Spot-photograph 

 (Fig. 16) op Zinc Blende. 



