690 Prof. R. W.Wood on Resonance Spectra of 



vapour. It will be seen that under this condition practically 

 all of the light of the line is located in the strong middle 

 component which bisects absorption-lines 3 and 4. It will 

 be noticed, however, that the centre of gravity of the line 

 lies a little nearer line 3, whereas in the case of the Cooper- 

 Hewitt lamp which operates at a low temperature with about 

 3 amperes, the green line exactly bisects the absorption-lines, 

 as is shown by the photograph S, Plate XVIII. (upper 

 spectrum the green line, lower spectrum iodine absorption). 

 The green line of the Cooper-Hewitt arc seen " end-on " 

 is shown by spectrum P, Plate XVII I., together with the 

 iodine absorption-lines. In this case the companion lines 

 are enormously intensified in comparison with the middle 

 bright component, which is weakened by self-absorption. 

 The quartz arc, when first started, takes about 8 amperes, 

 and the vapour is undoubtedly denser than in the Cooper- 

 Hewitt lamp. This causes a slight apparent shift of the 

 centre of gravity of the line towards the region of shorter 

 wave-length, due to the brightening of faint components which 

 lie close to the main line on this side. The green line of the 

 quartz lamp at low temperature is shown in coincidence with 

 the same line at high temperature by spectrum Q, PI. XVIII., 

 and the high temperature line in coincidence with the iodine 

 absorption-lines by spectrum T. Spectrum U shows the line 

 at medium temperature together with the absorption-lines. 

 These photographs were all made with the 40-foot spectro- 

 graph in the fourth order. Going back now to diagram N, 

 Plate XVI L, we see that the exciting line may be made to 

 assume a variety of intensity distributions according to the 

 temperature and current consumption of the lamp. At 

 medium temperatures we find two dark lines within the 

 broadened emission-line (next to last diagram N). The left- 

 hand one of these I find results from reversal of the central 

 component, and it persists as the temperature rises, as shown 

 by the lower diagram. The right-hand one is the dark gap 

 between the main line and the first component to the right, 

 and this one gradually fades out as the temperature rises. 



It is evident that much will be learned when photographs of 

 the resonance spectra are secured with the iodine vapour 

 excited by the line under all of these different conditions. At 

 present we have at our disposal only the resonance spectra 

 excited by the Cooper-Hewitt lamp, and the quartz lamp at 

 high temperature. The change in the appearance of the four 

 groups of resonance lines is shown in fig. 1 (p. 687), which 

 was drawn from the original negatives used for the prepara- 

 tion of spectra F and G, Plate XVII. The most conspicuous 



