434 Dr. W. Huggins and Mrs. Huggins. Relative Behaviour 



observable, tke strong blae line as well as the other lines of this 

 metal are very seldom, seen. In his table of the chromo spheric lines 

 Professor Young gives for the frequency of this strong blue line the 

 small number 3 ; while for the frequency of H and K, he gives 

 respectively the high numbers 75 and 50. 



From 1863, when I mapped the spectrum of calcium with a strong 

 spark from metallic calcium* I have constantly used the lines of 

 calcium as a comparison spectrum in stellar work. The experience 

 was familiar to me that as the quantity of calcium salt on the elec- 

 trodes became very small, H and K continued strong even when the 

 other calcium lines had almost disappeared. The suggestion then 

 occurred to me, that this behaviour of the lines might furnish a clue 

 to the phenomena which take place near the sun's limb. 



We were encouraged to use this experience as a guiding thought 

 in the experiments about to be described, by the consideration that 

 in the higher solar regions, where H and K appeared alone of the 

 calcium lines, the density must be much less than at the lower level of 

 the reversing layer. It seemed very probable that in the simple fact 

 of difference of density, lay the true explanation of the modifications 

 of the calcium spectrum as they are presented to us in solar and. 

 stellar phenomena. 



The problem before us was, therefore, to find out by experiments 

 an the laboratory, under what conditions the lines of calcium other 

 than the lines H and K, and in particular the strong blue line at 

 4226'9, were so greatly enfeebled relatively to H and K, that they 

 became quite insignificant, or even disappeared altogether from the 

 spectrum, leaving the very simple spectrum of the two lines H and 

 K, or nearly so. 



Professor Lockyer states that : — " Some of the substances which 

 have been investigated, including iron, calcium, and magnesium, 

 have probably a definite spectrum, consisting of a few lines, which 

 can only be completely produced at a temperature higher than any 

 which is at present available in laboratory experiments." ('Roy. 

 Soc. Proc.,' vol. 61, p. 205.) 



In the case of calcium : — 



" (4) A spectrum consisting of the two lines at 3705'18 and 

 3737*08 and the H and K lines, corresponding to a temperature 

 higher than the average temperature of the spark, as before ex- 

 plained." {Ibid., p. 161.) 



Such a spectrum was not actually obtained, but experiments with 

 a large intensity coil suggested that by a still greater increase of 

 intensity of the spark such a simple spectrum might appear. The 

 intensity of the strong blue line was reduced to one half of H and K. 

 {Ibid., Table, p. 162.) 



* < Phil. Trans.,' 1864, p. 139. 



