PhotoarajyJiic Reversals in Spectrum Photor/raj-ths. 577 



A olanco at the Plato will show tbat the absorption duo to 

 the presenco of iroii-Ya})our ttc. has resulted in a diminution 

 of intensity, ])articularly towards the violet end of the 

 spectrum. The phenomenon to which we wish to draw 

 attention is independent of this eifect and refers rather to the 

 results which are observed when the spectrum is studied in 

 detail. It is found that the simplification is ])urely selective 

 in its occurrence, for whereas certain of the lines appear in 

 this vacuum glow-spectrum with their characteristic intensity, 

 neighbouring lines of almost equal importance in tlie ordinary 

 arc and self-induction spark have so greatly diminished as to 

 be quite invisible. The effect, therefore, is in no way 

 connected with the actual exposure of the photograph or 

 intensity of the light. 



The present experiments are not sufficiently complete to 

 form any general theory on the effect of high pressures. 

 They indicate, however, that work in this direction would 

 yield results of some spectroscopical interest. 



With regard to the vacuum discharge, it would be of 

 importance to extend the work to other metals and ascertain 

 if the simplification found in the case of iron can be considered 

 as general. 



Our sincere thanks are due to Professor Arthur Schuster 

 for the valuable advice he has frequently given during the 

 course of this work; we desire also to record our indebtedness 

 to Dr. Gr. A. Hemsalech, who has kindly assisted in several 

 of the experiments. 



Physical Laboratories, 

 Owens College, Manchester. 



LXXIII. Photograpliic Reversals in Spectrum Photographs. 

 Bg E.. W. Wood, Professor of Experimental Physics, 

 Johns Hopkins University^, 



[Plate XXVI.] 



THE importance of distinguishing between photographic 

 and true reversals of lines in spectrograms has made 

 it seem worth while to investigate with some care the con- 

 ditions under which reversals due entirely to photographic 

 action can occur. Professor Trowbridge has advanced the 

 theory of selective reversibility of the silver salts in the sensi- 

 tive film for certain wave-lengths, and it was in part to 

 determine whether the tendency of a line to reverse was a 



* Comraunicated by the Author. From the Astrophysical Journal 

 for June 1903. 



