Precision-measurements in the X-Ray Spectra. 649 



realized, and which allows a considerable degree of accuracy 

 in measuring the reflexion-angles, is shown in figs. 1 and 2. 

 As is seen from the figure, the crystal-table is in fixed con- 

 nexion with a high-precision circle-scale which can be turned 

 in relation to the slit- and plate-holder. 



Fiff. 2. 



To fix the crystal in the required position and for micro- 

 metrical adjustments, the circle is provided with a tangent- 

 screw attached to the arm for the plate-holder. Crystal- 

 table and plate-holder as a whole can finally be turned 

 about the same axis. A circle-scale and nonius on the back 

 of the spectrograph serve to give an approximate adjustment 

 of the crystal to receive the X-ray pencil under the reflexion- 

 angle desired. The plate-holder and slit-stand are mounted 

 on the same bed-plate, and a thick lead cover laid over to 

 prevent blackening through diffused rays. 



The incoming pencil passes to broad slits of lead which 

 have in a later construction been supplied with micrometer- 

 screws to facilitate the necessary adjustments of spectrograph 

 and X-ray source. 



As a spectrograph of this type seemed to require a rather 



