338 E. W. G. Wyckoff— Crystal Structures of 



the maximum effect would no longer remain at the critical 

 absorption value for silver but would shift towards the 

 shorter wave lengths, more as the voltage is raised. 



It is of interest to inquire how closely these relative 

 absorption curves for various voltages approach to a 

 curve for the effect of an X-ray beam upon a photograph 

 plate which would be ideal for Laue photographs. Such 

 a curve would start from zero, rise quite rapidly to a de- 

 cided maximum (in actual practice this would be the crit- 

 ical silver value (A = 0.49) and immediately fall off to a 

 negligible value. Monochromatic radiation, on the one 

 hand, is not desired because so few planes would be in a 

 position to reflect the particular wave length for one set- 

 ting of the crystal ; on the other hand, if the curve rises 

 but slowly over a considerable range of wave lengths, 

 many planes will produce weak spots because they are 

 reflecting X-rays which are not particularly intense. If, 

 again, there is a considerable effect upon the plate for 

 wave lengths longer than wA=0.49, there will be produced 

 a considerable number of useless reflections. The result 

 in either of these last two cases is simply that more work 

 is required to interpret the Laue patterns because of the 

 presence of a large number of not particularly valuable 

 reflections. There should be no radiation shorter than 

 half the critical value for silver 



0-*9 A. IT. = , 24 , A ■ 



for otherwise it would be impossible to use the values in 

 the neighborhood of wA = 0.49 as assuredly of the first 

 order. This state of affairs is most nearly met by the 

 curve for 50,000 volts. At appreciably higher voltages 

 there is radiation in the wave lengths shorter than 0.25 

 A.U. ; if, again, lower voltages are used, the energy in the 

 wave lengths around 0.49 A.U. becomes small so that a 

 very long exposure is necessary and the second useless 

 maximum becomes relatively of great importance. Ex- 

 cept at voltages too low to be useful, the relative amount 

 of energy in the wave lengths about the critical absorption 

 of bromine is so small that the break in photographic 

 effect at this point is negligible. 



It was thought that the use of -filters might furnish a 

 more ideal curve. Such screening might be employed 

 either with a higher voltage to cut down the extremely 



