August 5, 1897 J 



NATURE 



319 



sequence of a much higher temperature, then we must 

 consider that the absorptions which define the various 

 star groups are more conditioned by the temperatures of 

 the hottest regions merely than by the thickness of the 

 absorbing atmospheres, or by the densities of the various 

 vapours. Another consideration to be borne in mind is 

 that if the atmospheres are in part composed of con- 

 densable vapours, and not entirely of gases permanent 

 at all stellar temperatures, condensation must always be 

 going on outside m the region of lowest temperature. 



Hence it is important to consider the conditions of 

 that part of the sun's atmosphere where it is known 

 beyond all question that certain, but not all, of the 

 absorptions which produce the Fraunhofer lines take 

 place. 



In my paper on the eclipse of 1893 {Phil. Trans., 1896, 

 vol. clx.xxvii. A, p. 603), I referred at length to this point. 

 The matter is so important that I do not hesitate to quote 

 in the present connection what I then said : 



"As a result of solar spectroscopic observations, combined 

 \N ith laboratory work, Dr. Frankland and myself came to the 

 cuaclusion, in 1869, that at least in one particular, Kirchhoffs 

 theory of the solar constitution required modification. In that 

 year we wrote as follows (/\oy. Soc. Proc, vol. xvii. p. 88) : — 



" ' May not these facts indicate that 

 the absorption to which the reversal of 

 the spectrum and the Fraunhofer lines 

 are due takes place in the photosphere 

 itself, or extremely near to it, instead 

 of in an extensive outer absorbing 

 atmosphere ? ' 



"In an early observation of a pro- 

 minence on April 17, 1870, I found 

 hundreds of the Fraunhofer lines bright 

 at the base, and remarked that a ' more 

 convincing proof of the theory of the 

 solar consdtution put forward by Dr. 

 Frankland and myself could scarcely 

 have been furnished' {Roy. Soc. Proc, 

 vol. xviii. p. 358). 



"During the eclipse of 1870, at the 

 moment of disappearance of the sun, a 

 similar reversal of lines was noticed ; 

 we had, to quote Prof. Young, 'a 

 sudden reversal into brightness and 

 colour of the countless dark lines of 

 the spectrum at the commencement of 

 totality.' On these observations was 

 based the view that there was a region 

 some 2" high above the photosphere, 



which reversed for us all the lines visible in the solar spectrum ; 

 and on this ground the name ' reversing layer ' was given to it. 

 "Continued observations, however, led me, in 1873, *^° 

 abandon the view that the absorption phenomena of the solar 

 spectrum are produced by any such thin stratum, and convinced 

 me that the absorption took place at various levels above the 

 photosphere. I need not give the evidence here ; it is set forth 

 m my ' Chemistry of the Sun ' (chap. xxii. pp. 303-309). On 

 the latter hypothesis the different vapours exist normally at 

 different distances above the photosphere, according to their 

 powers of resisting the dissociating effects of heat {Roy. Soc. 

 Proc, vol. xxxiv. p. 292). 



" My observations during the eclipse of 1882, in the seven 

 minutes preceding totality, to my mind set the matter at rest. 

 ' We begin with one short and brilliant line constantly seen in 

 prominences, never seen in spots. Next another line appears, 

 also constantly seen in prominences ; and now, for the first 

 time, a longer and thinner line appears, occasionally noted as 

 widened in spots ; while, last of all, we get, very long, very 

 delicate relatively, two lines constantly seen widened in spots, 

 and another line, not seen in the spark, and never yet recorded 

 as widened in spots' {ibid., vol. xxxiv. p. 297). 



"Similar olxservations in the same part of the spectrum were 

 made by Prof. Turner in \%%(i{Phil. Trans., 1889, vol. clxxx. A, 

 p. 391). His observations were made under less favourable 

 conditions than those in Egypt, and in the absence of statements 

 as to the relative lengths of the lines observed, it is impossible 

 to utilise them fully. 



NO. 1449, VOL. 56] 



"This is one of the most important points in solar physics, 

 but there is not yet a consensus of opinion upon it. Prof. 

 Young and others, apparently, still hold to the view first 

 announced by Dr. Frankland and myself in the infancy of the 

 observations, that the Fraunhofer absorption takes place in a 

 thin stratum, lying close to the photosphere." 



I next proceeded to discuss the numerous photographs 

 obtained during the eclipse, and I gave a map showing 

 that there was only the slightest relation between the 

 intensities of the lines common to the Fraunhofer and 

 the eclipse spectrum, and, further, that only a few of the 

 Fraunhofer lines are represented at all. Not only this, 

 but in the eclipse photographs there are many bright 

 lines not represented at all ainong the Fraunhofer lines. 



The chromosphere, then, is certainly not the origin of 

 the Fraunhofer lines, either as regards intensity or number. 

 From the eye observations made since 1868, I pointed 

 out many years ago that there is evidence that the 

 quiescent chromosphere spectrum indicates a higher 

 temperature than that at which much of the most valid 

 absorption takes place ; in other words, that the majority 

 of the lines associated with lower temperature are pro- 

 duced above the level of the chromosphere ; while the 

 eclipse photographs of 1893 ^"d 1896 afford evidence 



Fig. 10. — Tacchini's observations of the iron lines at 4924'! and 5oi8'6 on the spectrum of the quiet 

 chromosphere in 1872. It will be observed that new prominence lines were recorded when the iron 

 lines disappeared. 



by the greater length of some of the lower temperature 

 lines that we need not locate the region which produces 

 them at any great height above the chromosphere. 



The solar evidence, then, is that most of the line ab- 

 sorption is produced in, and not very far above, the 

 chromosphere. This is a conclusion we are bound to 

 accept in a discussion of the origin of stellar absorption 

 in the absence of evidence to the contrary. We have 

 no right to assume that the absorption will be produced 

 at the top of the atmosphere in one star, and in the 

 bottom in another, when the atmospheres are once re- 

 latively quiescent. 



Quite recent work has very greatly strengthened these 

 conclusions in regard to the sun. The conclusion with 

 regard to the high temperature of the quiescent chromo- 

 sphere depended chiefly upon the Italian observations 

 and upon investigations communicated by myself to 

 the Royal Society in 1879 {Roy. Soc. Proc, 1879, vol. xxx. 

 p. 22), and 1881 {ibid., 1881, vol. xxxii. p. 204), on the 

 effect of high-tension electricity on the line spectra of 

 metals (Fig. 10). 



These investigations consisted in noting (i) the lines 

 brightened in passmg a spark in a flame charged with 

 metallic vapours, and (2) the lines brightened on passing 

 from the arc to the spark. It was found, in the case of 



