798 



SCIENCE 



[N. S. Vol. XL. No. 1040 



these cases and the reflection of X-rays by a 

 crystal. Suppose for example that a pencil 

 of homogeneous X-rays meets the cube face of 

 such a crystal as rocksalt. The atoms of the 

 crystal can be taken to be arranged in planes 

 parallel to that face, and regularly spaced. If 

 the rays meet the face at the proper angle, 

 and only at the proper angle, there is a reflected 

 pencil. It is to be remembered that the re- 

 flection is caused by the joint action of a 

 series of planes, which, in this case, are paral- 

 lel to the face; it is not a reflection by the 

 face itself. The face need not even be cut 

 truly: it may be unpolished or deliberately 

 roughened. The reflection takes pla.ce in the 

 body of the crystal, and the condition of the 

 surface is of little account. 



The allotment of the atoms to a series of 

 planes parallel to the surface is not of course 

 the only one possible. For example in the 

 case of a cubic crystal, parallel planes con- 

 taining all the atoms of the crystal may also 

 be drawn perpendicular to a face diagonal of 

 the cube, or to a cube diagonal, or in many 

 other ways. We may cut the crystal so as 

 to show a face parallel to any series, and then 

 place the crystal so that reflection occurs, but 

 the angle of incidence will be different in each 

 case since the spacings are different. It is not 

 necessary to cut the crystal except for con- 

 venience. If wave-length, spacing and angle 

 between ray and plane are rightly adjusted to 

 each other, reflection will take place in the 

 crystal independently of any surface arrange- 

 ment. 



This is the " reflection " method of explain- 

 ing the Laue photograph. W. L. Bragg showed 

 in the first place that it was legitimate, and in 

 the second that it was able to explain in the 

 position of all the spots which Laue found 

 upon his photographs. The different spots are 

 reflections in different series of planes which 

 may be drawn to contain the atoms of the 

 crystal. The simpler conception led at once to 

 a simpler procedure. It led to the construc- 

 tion of the X-ray spectrometer, which re- 

 sembles an ordinary spectrometer in general 

 form, except that the grating or prism is re- 

 placed by a crystal and the telescope by an 



ionization chamber and an electroscope. In 

 use a fine pencil of X-rays is directed upon 

 the crystal, which is steadily turned until a 

 reflection leaps out ; and the angle of reflection 

 is then measured. If we use different crystals 

 or different faces of the same crystal, but keep 

 the rays the same, we can compare the geomet- 

 rical spacings of the various sets of planes. 

 If we use the same crystal always, but vary 

 the source of X-rays, we can analyze the latter, 

 measuring the relative wave-lengths of the 

 various constituents of the radiation. 



We have thus acquired a double power: (1) 

 We can compare the intervals of spacing of 

 the atoms of a crystal or of different crystals, 

 along various directions within the crystal; in 

 this way we can arrive at the structure of the 

 crystal. (2) We can analyze the radiation of 

 an X-ray bulb; in fact we are in the same 

 position as we should have been in respect to 

 light if our only means of analyzing light had 

 been by the use of colored glasses, and we had 

 then been presented with a spectrometer, or 

 some other means of measuring wave-length 

 exactly. 



We now come to a critical point. If we 

 knew the exact spacings of the planes of some 

 one crystal, we could now by comparison find 

 the spacings of all other crystals, and measure 

 the wave-length of all X-radiations ; or if we 

 knew the exact value of some one wave-length, 

 we could find by comparison the values of aU 

 other wave-lengths, and determine the spacings 

 of all crystals. But as yet we have no abso- 

 lute value either of wave-length or of spacings. 



The difficulty appears to have been over- 

 come by W. L. Bragg's comparison of the re- 

 flecting effect in the case of rocksalt or sodium 

 chloride, and sylvine or potassium chloride. 

 These two crystals are known to be " iso- 

 morphous " ; they must possess similar arrange- 

 ments of atoms. Yet they display a striking 

 difference both in the Laue photograph and on 

 the spectrometer. The reflections from the 

 various series of planes of the latter crystal 

 show spacings consonant with an arrange- 

 ment in the simplest cubical array, of which 

 the smallest element is a cube at each comer 

 of which is placed the same group, a single 



