160 



Mr. J. Norman Lockyer. 



(3) The spark spectrum, differing from the arc spectrum in the 

 enhancement of some of the short lines and the reduced brightness 

 of others. 



(4) A spectrum consisting of the lines which are intensified in the 

 spark. This we can conceive to be visible alone at the highest tem- 

 perature in a space efficiently shielded from the action of all lower 

 ones, since the enhanced lines behave like those of a metal when a 

 compound of a metal is broken up by the action of heat. 



A complete list of the iron lines seen at the different temperatures 

 would be too long to reproduce here, so that the following statement 

 of intensities is limited to the lines enhanced in the spark. The be- 

 haviour of these lines under the different conditions of experiment 

 is as follows : — 



Lines of Iron which are enhanced in Spark. 



Ware 

 length. 

 (Eowland) . 



Intensity 

 in flame. 



Intensity in 

 are (K & E) 

 Max. = 10. 



Length in 



arc (L) 

 Max. = 10. 



Intensity in 

 spark (T). 

 Max. = 10. 



Intensity in 

 hot spark 



(L). 

 Max. = 10. 



4233-3 





1 







4 



4508-5 





1 







4 



4515-4 





1 







4 



4520-4 





1 







2 



4522-7 





1 



3 





4 



4549 -6 





4 



5 





6 



4584-0 





2 



4 





7 



4924-1 





1 



3 



6 



6 



5018 -6 





4 







6 



K & E = Kayser and Eunge, T = Thalen, L = Lockyer. 



The arc and spark spectra are photographically compared in 

 Plate 1, The photographs were taken with an instrument having 

 two prisms of 60°, a collimator of 3-in. aperture and 5-ft. focus, and 

 a photographic lens of 19 in. focal length. The enhanced lines are 

 indicated at the bottom. It should be remarked that the enhance- 

 ment of the line at 4233'3 is not clearly seen with this dispersion, 

 but it is quite conspicuous in a photograph recently taken in the 

 first order spectrum of a Rowland grating of 21 ft. 6 in. radius. 



Calcium. 



I next proceed to consider the results obtained in the case of 

 calcium. 



Among the chief observations of the spectrum of this metal are 

 those made by Thalen, Kayser and Runge, and myself. 



Thalen chiefly confined himself to a study of the spectrum at the 



