PHYSICS IN 1913 611 



maxima at definite angles. These maxima correspond to homo- 

 geneous rays of wave-length X, given by nX = 2d sin 0, where 

 is the angle of incidence, n an integer, and d the distance 

 between successive planes, for only at such an angle will the 

 waves reflected from the successive planes reinforce one another. 

 Some of the radiation from an X-ray bulb is " white " radiation, 

 and accordingly contains components reflected at any angle ; 

 in addition there are strong homogeneous beams reflected only 

 at certain angles, the angle depending on the wave-length of 

 the particular radiation in the way described. The homogeneous 

 radiations are the " characteristics " of the metal of the anti- 

 cathode, investigated by Barkla by measurements of their absorp- 

 tions ; the target, or anticathode used by the Braggs in their 

 earlier experiments was of platinum, and they determined the 

 wave-length of the characteristics of platinum from the formula 

 already given, d being worked out from the weight of the atom, 

 the assumed structure of the crystal, and its density. 



Moseley and C. G. Darwin about the same time examined 

 the radiation from a tube with a platinum target by reflecting 

 it from the principal cleavage-planes of different crystals, rock- 

 salt, selenite, and potassium ferrocyanide being used. They 

 detected the reflected beam by means of the ionisation produced, 

 and showed that the primary and reflected beams contained 

 the same constituents, present, however, in different proportions 

 in the two beams, in other words that the crystal did not 

 manufacture a special type of radiation, but picked out radia- 

 tions already present. They detected five homogeneous 

 radiations, reflected at definite angles from each of the 

 crystals, and measured each radiation in three different orders, 

 that is, they found for each radiation successive values of 0, 

 the angle of reflection, corresponding to the values 1, 2, 3 

 for n in the formula nX — 2d sin 0. The reflection theory was 

 strongly confirmed both by comparing the results with the 

 different crystals, which showed d a constant for the given 

 face in each crystal, but differing from crystal to crystal, and 

 by the obtaining of each line in different orders. Thus a 

 homogeneous X-radiation is reflected from a crystal plane 

 rich in atoms at certain definite angles, whose sines are simple 

 multiples of one another ; this corresponds to the different 

 orders in the grating spectra of visible light. To the grating 

 space corresponds not the distance between the atoms in a 



