Reflexion of X Rays by Crystals. 



891 



A sheet of palladium foil was placed over the slit. The 

 foil cut down the intensity of the principal rhodium line to 

 half value and almost destroyed all the radiations of shorter 

 wave-length than the two rhodium lines. A block of 

 aluminium was added, which still further reduced the 

 principal rhodium line to one seventh. When the primary 

 rays had passed through these screens, the principal rhodium 

 line had fallen to one fourteenth of its first value, the lesser 

 rhodium line would be reduced about half as much, and the rest 

 of the radiation would be very largely absorbed. The primary 

 rays entering the ionization chamber directly then caused 

 a leak of 178 in one beat of the clock. The reflexion current 

 of the first order in the 100 face amounted to 676, when the 

 crystal was turned through five minutes of arc for each beat 

 of the clock. If the primary beam had consisted of the 

 principal line of rhodium only, these figures would have 

 been sufficient to give an absolute value. I hope to give 

 such absolute values in a little time. 



The effects of temperature upon the intensity of reflexion 

 have been investigated by surrounding the crystal by a small 

 electric oven. Mica windows permitted the passage of the 

 X rays and stopped convection currents. The results are 

 most conveniently expressed in terms of the ratio of the 

 intensity I at the temperature of the room to the intensity I' 

 at the temperature of the furnace. The temperature was 

 measured with sufficient accuracy by inserting into the oven 

 a thermometer the bulb of which was nearly in contact 

 with the crystal. 



The ionization currents were observed for each separate 

 position of the crystal. The figure shows the second and 



Fig. 5. 



12 13.30 |8°30' 20° 



third spectra of the rhodium line at each of the two tem- 

 peratures. The diminution of intensity with temperature is 



