494 Dr. Marine Siegbahn and Dr. Einar Friman on 



Similarly j in tbe case o£ a double mirror, 



^ = 0,0^, 



if is taken on the common edge of the component mirrors. 

 Thus, in the case of any multiple mirror, we have, as before,. 



r'={2r, n=na;, 



where Hi ■= 1 — 2n; . n,-, provided that all the component 

 mirrors have a common point of intersection, and that this 

 point is taken as the origin of the vectors r, r'. 



Under these circumstances, therefore, the treatment of 

 point-objects and their images is formally the same as that 

 of incident and reflected rays. 



September 5, 1916. 



L1I. On an X-B.ay Vacuum Spectrograph. By Manne 

 Siegbahn, Dr. phil., and Einar Friman, Dr. phil* 



IN order to examine the high frequency spectra of the 

 elements by long wave-lengths the authors have had 

 a vacuum spectrograph built. Hereby, as our former 

 measurements t have shown, the following conditions must 

 be satisfied. First, the crystal must be movable, as otherwise 

 irregularities in the structure of" the crystal may be of great 

 influence (comp. Rutherford and Andrade and E. Wagner). 

 Secondly, in order to get a good resolving power, besides 

 using a fine slit it is of great importance to focus the rays. 



The apparatus built on this principle is shown in fig. 1, in 

 both horizontal and vertical section. The spectrograph 

 consists of a round metallic box of 6 mm. thickness, a height 

 of 8 cm. and an inner diameter of 30 cm. The upper part 

 BB, 3*5 cm. broad, is carefully plane-ground, as well as the 

 corresponding part CC of the cover D. This as well as the 

 bottom is furnished with radial reinforcements in order to 

 resist the pressure better. The cover has a handle and a 

 screw, the latter being used to lift it after the air has been 

 admitted. The box is supported by three set-screws. In the 

 middle of the bottom there is a conical hole Avith a metallic 

 cone E, fitting well in it. This cone, being kept in the hole 

 by a ring screwed into the box. after lubricating; can be turned 

 without the air passing through. The crystal table, placed 



* Communicated by the Authors. 



+ Phil. Mag. vol. xxxi. p. 403 (1916) ; vol. xxxii. p. 39 (1916). 



