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SCIENCE PROGRESS 



(4) Crystal removed ; short exposure to other half of the 

 primary beam passing through the slit. 



Measurement of the images so obtained gave at once the 

 angle of reflection required. This is, of course, the important 

 measurement. It can be found by determining two points on 

 the direct line of the primary beam and two points on the re- 

 flected beam. In the method described this is exactly what 



Fig. 2. 



The pencil XX is reflected at the crystal C. The photographic plate is first placed at P'P' to get the 

 points R' and L', and afterwards at PP to get the points R and L. The exact distance between the plates 

 being known it is possible to get an accurate measure of the angle between L'L and R'R. 



is done. In some cases it has been assumed that one point 

 on each line is in the face of the crystal ; but since the X-rays 

 actually penetrate the crystal the position of this point is 

 not sufficiently definite, unless a very thin crystal is used and 

 carefully placed in its right position. 



In the ordinary spectrometer the similar case does not 

 arise, because the object glass of the telescope sends all parallel 

 rays to the same focus, so that the position of that focus deter- 



