SOLAR CHANGES OF TEMPEllATURE. 175 



experienced during the century, suggested to us the desirability of 

 reconsidering the c^uestion, especially as we have now some new fac- 

 tors at our disposal. These have been revealed by the study, now 

 extending over twent}^ years, of the widened lines in sunspots, which 

 suggested the view that two effects ought to be expected in a sunspot 

 cycle instead of one. 



THE WIDENED LINES. 



It will be gathered from previous communications to the Royal 

 Society ^ that, on throwing the image of a sunspot on the slit of a spec- 

 troscope, it is found that the spectrum of a spot so examined indicates 

 that the blackness of the spot is due not only to general, but to select- 

 ive absorption,^ and that the lines widened by the selective absorption 

 vary from time to time. 



Since the year 1879 the selective absorption in spots has been 

 observed for every spot that was large enough to be spectroscopically 

 examined, the method adopted being as follows: 



The regions of the spectrum investigated lie between F — h and h — D, 

 and an observation consists in observing the six most widened lines in 

 each of these regions. These lines are then identified on the best solar 

 spectrum maps available and their wave lengths determined. 



An examination of many years' records of these widened lines has 

 shown that at some periods they are easily traceable to known ele- 

 ments, while at others their origins have not been discovered, so the 

 latter have been classed as '"unknown" lines. If we compare these 

 two periods with the sun-spot curve as constructed from the measure- 

 ments of the mean spotted area for each j^ear, it is found that when the 

 spotted area is greatest the widened lines belong to the "unknown" 

 class, while when the spotted area is least they belong to the "known" 

 class. 



The majority of the lines traced to some terrestrial origin belong to 

 iron, but the lines of other elements, such as titanium, nickel, vana- 

 dium, scandium, manganese, chromium, cobalt, etc., are also repre- 

 sented in a less degree. 



It is quite likely that some of the "unknown" lines are higher tem- 

 perature (enhanced) lines of known chemical elements. 



In our laboratories we have means of differentiating between three 

 stages of temperature, namely, the temperature of the flame, the 

 electric arc, and the electric spark of the highest tension. At the 

 lowest temperature, that of the flame, we get a certain set of lines; a 

 new set is seen as the temperature of the electric arc is reached. At 

 the temperature of the high-tension spark we again have many new 



^ Proc. Roy. Soc, vd. xl, p. 347, 1886; vol. xlii, p. 37, 1887; vol. xhd, p. 385, 1889; vol. 

 lvii,p. 199, 1894. 

 ^ P. R. S., Lockyer, 1866, October 11. 



