314 Prof. W. L. Bragg and Messrs. James and Bosanquet : 



surface, the effect of which will be discussed later. It can 

 be shown that, although they differ greatly, their mean 

 represents accurately the strength of reflexion if the face 

 were cut true. In taking the readings, the crystal was 

 turned 5 minutes of arc for every beat of a metronome, 

 beating 100 to the minute. 



Comparison of (311) L, (311) R, and (300) R. 



Face. 

 (311) L 



Sweep of 



crystal. 



O / O / 



8 50-11 20 



Chamber Potentio- 

 angle. meter 

 t scale. 

 20 50 2 



Readings. 



(71,73,73,72) 



Mean of 

 readings. 



72-2 



(311) R 



10 05-12 35 



2100 



2 



(57,55,58,57,56,56) 



56-5 



(311) L 



8 50-11 20 



20 50 



2 



(73,74,74,75) 



740 



(300) R 



17 30-20 00 



38 25 



3 



(77, 78, 78, 78) 



77-8 



(311) L 



8 50-1120 



20 50 



2 



(72,71,70,72,71,70) 



71-0 



(300) R 



17 30-20 00 



38 25 



3 



(78, 78, 79, 80, 80) 



79-0 



A survey of the three reflexions showed that the homo- 

 geneous radiation was responsible for 76*9 per cent, of the 

 total effect in the case of the (300) R reflexion, 33*0 per cent, 

 for the (311) R, and 32'2 percent, for the (311) L reflexion. 

 Since the intensity is very much greater for the (300) face 

 than for the (311) face, different scales on the potentiometer 

 were used. A reading of 72'2 on the second scale represents 

 72'2 per cent, of a total voltage of 15'72, the corresponding- 

 voltage for the third scale being 22*79. 



Taking this into account and allowing for the general 

 radiation, one gets a ratio 



Mean intensity, face (311) _ 3*22 _ ri . 9 oq- 

 Intensity, face (300) R ~ l3 T 45 ~ [j ' : ' d ^^' 



In another experiment, (300) R and (300) L were com- 

 pared, and in this way the relative mean intensities of (311) 

 and (300) measured. 



In order to have a uniform system of indicating both the 

 order of reflexion and the face at which it is taking place, 

 the convention of multiplying the indices of the face by the 

 order has been adopted. Thus, by the reflexion from (622) 

 is meant the second order of reflexion from the face (311). 



The crystal is not turned continuously during each reading; 

 its setting is altered five minutes of arc at each beat of a 

 metronome by means of a series of spokes on the tangent 

 screw. It would be preferable to turn the crystal with a 

 uniform angular velocity, but it is unlikely that any ap- 

 preciable error was caused by the method used. In order 



