io8 



NATURE 



[November 29, 1900 



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 spectroscope, it is found that the 

 spectrum of a spot so examined indicates that the black- 

 ness of the spot is due, not only fto general, but to 

 selective 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 — b and b — 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 elements, 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 sunspot curve as constructed from the measure- 

 ments of the mean spotted area for each year, 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, vanadium, scandium, man- 

 ganese, chromium, cobalt, &c., are also represented in a 

 less degree. 



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

 higher temperature (enhanced) lines of known chemical 

 elements. 



In our laboratories we have means of differentiating 

 between three stages of temperature, namely, the tempera- 

 ture 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 lines, called enhanced lines, added, while 

 many of the arc lines wane in intensity. 



It is found that at sunspot minimum, when the " known " 

 lines are most numerous, the lines are almost invariably 

 those seen most prominent in the arc. Passing from 

 the sunspot minimum towards the maximum the 

 "unknown" lines gradually obtain the predominance. 

 As said before, they may be possibly " enhanced lines " 

 — that is, lines indicating the action of a much higher 

 temperature on known substances. 



Unfortunately the records of enhanced lines at South 

 Kensington, having been obtained from photographs, are 

 chiefly confined to a region of the spectrum not covered by 

 the visual observations of widened lines in sunspot spectra. 



We can only point to the evidence acquired in the 

 case of one metal — iron, for which photographs of the 

 enhanced lines in the green and yellow parts of the 

 spectrum have been obtained. 



This evidence quite justifies the above suggestion, for 

 the enhanced lines of iron can be seen revealing them- 

 selves as the number of unknown lines increases. 



We are, therefore, quite justified in assuming a very 

 great increase of temperature at the sunspot maximum 

 when the "unknown " lines appear alone. 



The curves of the " known " and " unknown " lines 

 have been obtained by determining for each quarter of a 

 year the percentage number of known and unknown lines 

 and plotting these percentages as ordinates and the time 

 elements as abscissas. Instead of using the mean curves 



1 Proc. Roy. Soc, vol. x1. p. 347, 1886 ; vol. xlii. p. 37, 1887 ; vol. xlvi. 

 p. 385, 1889; vol. Ivii. p. 199, ir94. 

 '>■ P.R.S., Lockyer, 1866, October 11. 



NO. 1622. VOL. 63I 



for all the known elements involved, that for iron is em- 

 ployed, as it is a good representative of " known " elements, 

 and has been best studied. When such curves have been 

 drawn they cross each other at points where the per- 

 centage of unknown lines is increasing, and that of the 

 iron or known lines are diminishing, or vice versa. 



We seem, therefore, to be brought into the presence of 

 three well-marked stages of solar temperature. 



When the curves of known and unknown lines cross 

 each other, that is, when the number of known and un- 

 known lines is about equal, we must assume a mean 

 condition of solar temperature. When the unknown 

 lines reach their maximum we have indicated to, us a 

 + pulse or condition of temperature. When the known 

 lines reach their maximum we have a - pulse or con- 

 dition of temperature. 



The earliest discussion showed that, generally speaking, 

 the unknown-lines curve varied directly, and the iron- 

 lines curve varied inversely with the spot-area curve. 

 The curves now obtained for the whole period of twenty 

 years not only entirely endorse this conclusion, but 

 enable more minute comparisons to be drawn. 



The " widened line " curves are quite different from 

 those furnished by the sun-spots. Ascents and descents 

 are both equally sharp, changes are sudden, and the 

 curves are relatively flat at top and bottom. The 

 crossings are sharply marked. 



During the period since 1879 three such crossings 

 have occurred, indicating the presence of mean solar 

 temperature conditions,in the years 1881,1886-7,^ and 1892. 

 It was expected that another crossing with the knov/n 

 lines on the rise would have occurred in 1897, indicating 

 thereby the arrival of another mean condition of solar 

 temperature, but as yet no such crossing has taken place. 



The following tabular statement shows the years of those 

 crossings, together with the probable dates, in brackets, 

 of the two previous crossings, as determined by the time 

 of occurrence of the preceding sun-spot maximum. 



Rise of 



Unknown lines 

 Known lines 



(1869) 

 (1876) 



1886-7 



1892 



? 



Comparison of Solar and Terrestrial Weather. 



It has long been known that a cycle of solar weather 

 begins in about lat. 32° N. and S., and in a period of 1 1 

 years ends in about lat. 5° N. and S. 



Just before one cycle ends another commences. The 

 greatest amount of spotted surface occurs when the solar 

 weather-changes produced in the cycle reach about 

 lat. 16° N. and S. 



It becomes, therefore, of the first importance to corre- 

 late the times of mean solar temperature, and of the -I- 

 and - heat pulses, with the solar weather cycle, in order to 

 arrive at the temperature-history of the sun during the 

 period which now concerns us. This may be done as 

 follows : — 



1 According to the observations the mean was reached in December i8 

 or January 1887. 



