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Plates secured with many lines of various elements will have to l>c 

 compared. The mighty 30-foot spectroheliograph of the "tower 

 telescope" of Mount Wilson is excellently adapted to work of this 

 kind, not only on account of its great dispersion permitting the use 

 of finer lines, but chiefly because it is provided with two camera 

 slits, so that perfectly simultaneous photographs with different lines 

 may be secured. By this arrangement, really comparable mono- 

 chromatic pictures of the sun are obtained, since the otherwise con- 

 fusing influence of the variable refraction in our atmosphere is thus 

 rendered harmless. 



I feel greatly obliged to Prof. George E. Hale for having procured 

 for me the opportunity of making an investigation at the Mount 

 Wilson Solar Observatory, but more still for his keen and stimulating 

 interest in the problems, suggested by the application of the prin- 

 ciple of anomalous refraction in astrophysics. I am also very much 

 indebted to the kindness of Mr. F. Ellerman, Mr. W. S. Adams 

 and Dr. Ch. M. Olmsted for valuable information and assistance in 

 connection with the inquiry here reported upon. 



Physics. — "The Zeem. \x-Ejrect of the strong lines of the violet 

 spark spectrum of iron in the region X 2380 — A 4416." By 

 Mrs. H. B. van Bildehbekk-van Meirs. (Communicated by 

 Prof. P. Zeeman). 



The concave Rowland grating used in the experiments here com- 

 municated has 14438 lines per inch, a width of 8 cm., and a 

 radius of curvature of 304.96 cm. The grating is mounted according 

 to Runge and Paschen's method. 



The spark passed between the iron poles of the magnet in the 

 direction of the line of force. It was originated by the discharge 

 of the secondary coil of a Ruhmkorff, a self-induction and condenser 

 being placed in parallel. 



Further details will be given in my thesis for the doctorate. 



The time of exposure varies from 30 to 120 minutes. 



In order to determine the field strength I made simultaneous ex- 

 posures of the iron and zinc spectra. The amount of separation of 

 the zinc line 4680.33 was compared with the result of the measu- 

 rements of Cotton and Weiss (Journal de Physique, June 1908), the 

 strength of field being supposed proportional to the amount of 

 separation. 



