34 



Sir Norman Lockyer. On the Spark 



[Jan. 31, 



Iron. 



Magnesium. 



4045 -98 4325 -94 



3829 '50 



4063 -76 4383 72 



3832 -45 



4071 '91 4404 '93 



3838 -44 



4271 93 4415 -29 





4308 -08 





As shown by the enlargements, this reversal is not always symmetrical 

 with the original bright line, and the part of the emission line on the 

 red side of the reversal is the brighter. It will be evident that in such 

 cases if the exposure is insufficient for the less intense component to 

 be photographed, the appearance of a bright line in a position greatly 

 displaced towards the red will be presented, as is shown in the line of 

 iron at X 4260-64. 



In the case of copper, we have stopped apparently at such an inter- 

 mediate stage, and the phenomena observed thus appear to agree more 

 closely with those described by Dr. Wilsing. In this case no reversals 

 have actually taken place, and the only lines seen in the water-spark 

 spectrum present the appearance of broad bands, considerably dis- 

 placed towards the red, and having their more refrangible edges rather 

 sharply denned by absorption, which is not otherwise manifested, while 

 the less refrangible edges are very diffuse. 



With zinc two of the lines in the strong group of three in the blue- 

 green region show reversal, the absorption line being nearly normal, 

 separating parts of the emission line of very different intensities. 

 These lines, A 4722*34 and X 481072, are much more intense on the red 

 side of the central absorption line. In the remaining line of the 

 triplet at X 4680*32 there is no reversal, but the maximum of intensity 

 of the emission line is also shifted towards the red. 



(b.) Classification of the Different Phenomena presented. 



Considering the photographs obtained with various exposures and 

 conditions, the phenomena observed may be grouped as follows : — 



(1.) Broadened bright line. 



(2.) Broadened bright line with central absorption line. 

 (3.) Broadened bright line with non-symmetrical absorption 

 (maximum of emission towards red). 



(1.) Broadened Bright Line. — This appearance is well shown in the 

 spectrum of copper and the under-exposed spectrum of iron. 



The broadened line is not of uniform intensity throughout its 

 breadth, being stronger on the blue side, which is terminated almost 

 abruptly, while the broader towards the red is more diffuse. 



