202 Professor William H. Bragg [June 5, 



cube, or to u cu])e diugonal, or in many other ways. \Ye 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 convenience. 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 arrangement. 



This is the " reflection " method of explaining the Lane photo- 

 graplj. W. L. Bragg showed in the first place that it was legitimate, 

 and the second that it w^as 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 construction of the X-ray 

 spectrometer, which resembles an ordinary spectrometer in general 

 form, except that the grating or prism is replaced 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 geometrical 

 spacings of the various sets of planes. If we use the same crystal 

 always, but vary the source of X-rays, w^e can analyse the latter, 

 measuring the relative wave-lengths of the various constituents of 

 the radiation. 



We have thus acquired a double powder : — 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 analyse 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 analysing light had 

 been by the use of coloured glasses, and w^e had then been presented 

 with a spectrometer, or some other means of measuring wave-length 

 exactly. 



We now come to a critical point. If w^e 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 all other wave-lengths, 

 and determine the spacings of all crystals. But as yet we have no 

 absolute value either of wave-length or of spacings. 



The difficulty appears to have been overcome by W. L. Bragg's 

 comparison of the reflecting effect in the case of rocksalt or sodium 

 chloride, and sylvine or potassium chloride. These two crystals are 

 known to be " isomorphous " ; they must possess similar arrangements 

 of atoms. Yet they display a striking difference both in the Laue 



