Hie Intensity of Reflexion of X-Rays by Rock-Salt. 313 



A series of readings plotted in this way is shown in fig. 1, 

 The readings are at first approximately constant, being due 

 to the general radiation. As the position at which the 

 chamber is set approaches that at which homogeneous rays 



Fig. 1. 



-O A- 



25 c 

 Chamber Angh 



2 7° 



are received, the ionization rises rapidly, remains constant 

 again as long as the whole pencil of homogeneous rays enters 

 the chamber, and then falls to a value approximately equal 

 to its former steady value when the homogeneous rays" are no 

 longer received. 



6. When comparing two crystal faces, this survey is made 

 in each case. One of the faces is then mounted in the 

 spectrometer, the chamber set so that it receives the homo- 

 geneous beam, and a series of readings taken by sweeping 

 the crystal backwards and forwards. The crystal faces are 

 interchanged, the chamber reset, and a series of readings 

 taken for the other face. This process is repeated several 

 times, and the means of the intensities for the faces are 

 compared. The preliminary survey indicates what fraction 

 of the total intensity observed must be subtracted, for each 

 face, in order to allow for the general radiation ; and when 

 this has been done, the ratio of the corrected readings gives 

 the ratio of the _ intensity of reflexion by the two faces. 

 A series of readings obtained in this way is given below. 

 It is a comparison of the reflexion by the (311) face of NaCl, 

 mounted so as to face left on the spectrometer, of the same 

 face turned through 180° so that it faces right, and of the 

 third-order reflexion from the face (100) mounted so as to 

 face right. The difference between the values for (311) L 

 and (311) R is due to inaccurate grinding of the crystal 



