696 Dr. A. E. H. Tutton [March 14, 



the trigonal space-lattice, and exhibits the threefold nature of the 

 axis of symmetry which is perpendicular to the plate and along which 

 the X-rays were directed. 



Professor Laue has also experimented with the crystals of a 

 number of other cubic substances, and, like zinc blende, they all 

 show holohedral symmetry about a tetragonal axis. 



W. L. Bragg has found that stronger photographs of the same 

 nature can be obtained from mica, using nearly gi'azing incidence, 

 and it is by use of this fact that Mosely and Darwin have been able 

 to study the reflected rays electrically, and found them to resemble 

 ordinary X-rays. By the kindness of Mr. Bragg, a diagram of his 

 apparatus and a positive lantern sHde of one of his mica spot photo- 

 graphs are exhibited on the screen. The large white spot is due to 

 the direct rays ; the next brightest but smaller spot considerably above 

 the direct one is due to rays reflected from the cleavage plane, and 

 the other spots to reflections from other possible faces and planes of 

 the monoclinic space-lattice ; all the spots are such as are compatible 

 with this lattice, and with the well known fact that the monoclinic 

 angle is almost 90°. The spots lie on two ellipses intersecting at the 

 central direct spot and at the upper bright one, each ellipse corre- 

 sponding to a set of parallel rows of points, the crossing of the two 

 sets forming rhombic parallelograms, the basal edges of the mono- 

 clinic prismatic space-lattice cell of mica. 



Incidentally these experiments appear likely to throw light on the 

 much-debated question of the nature of the X-rays. As all the 

 experiments unite in indicating that a fraction of the X-rays suffers 

 reflection at the planes of atoms parallel to the more important 

 possible crystal faces, all being planes of atomic points of the space- 

 lattice, it would appear that the X-rays are some type of wave-motion, 

 or at any rate some kind of pulse with an extended wave-front. Yet 

 after reflection they retain the same corpuscular character which Prof. 

 W. H. Bragg has shown they possess. For the liberation of a high- 

 speed electron from an atom traversed by the X-ray cannot be 

 explained, according to Rutherford, unless it be supposed that the 

 energy of the X-ray is concentrated over a minute volume, and can 

 be given up in an encounter with a single atom. Hence these 

 experiments show that the X-rays possess at the same time the 

 apparently opposite properties of extension over a wave-front and 

 concentration in a corpuscular point. 



It appears to the lectnrer that the simpler explanation is that we 

 are truly dealing with waves, but that the wave-lengths of the X-rays 

 are excessively short, approaching atomic dimensions. This view 

 that the X-rays are waves is further supported by the results of some 

 experiments just completed by Barkla, in which a diverging pencil 

 of X-rays was directed on a crystal of rock salt, and the issuing rays 

 received on a photographic plate in the same manner as in the 

 experiments already described. The developed plate shows a new 



