186 



KNOWLEDGE, 



[AufiUBT 1, 1898. 



to assert with poaitiveness that these innumerable lines 

 corresponded exactly with the dark lines of the spectrum," 

 yet that the general appearance and grouping of the lines 

 in the spectrum seemed perfectly familiar to him. Mr. Pye, 

 who observed the same eclipse and also saw the ■" flash," 

 says that the effect was "as if all the dark lines were 

 converted into bright ones." 



Spectroscopists have, as a rule, been content to accept 

 the "flash" as in ail probability practically a reversal 

 of the Fraunhofer lines. Sir Norman Lockyer, whilst 

 objecting to it, thus clearly states the ordinary view as to 

 the " reversing layer " ; — 



(1) We have terrestrial elements in the sun's atmosphere. 



(2) They thin out in the order of vapour density, all 

 being represented in the lower strata, since the tempera- 

 ture of the solar atmosphere at the lower levels is incom- 

 petent to dissociate them. 



(3) In the lower strata we have especially those of 

 higher atomic weight, all together forming a so-called 

 "reversing layer," by which chiefly the Fraunhofer spec- 

 trum is produced. (" Chemistry of the Sun," p. 303.) 



It follows that, on this view, the spectrum of the base of 

 the solar atmosphere should most resemble the ordinary 

 Fraunhofer spectrum {ibiiL, p. 306). In 1K73, however, 

 Prof. Lockyer was led to take an entirely different view, 

 and he was convinced " that the absorption took place at 

 various levels above the photosphere." (" Recent and 

 Coming Eclipses," p. 99.) " On this latter hypothesis, the 

 different vapours exist normally at different distances 

 above the photosphere, according to their powers of 

 resisting the dissociating effects of heat." It follows that 

 " the spectrum of the base should least resemble the 

 Fraunhofer spectrum, because at the base we only get 

 those molecules which can resist the highest temperatures." 



The immense importance of the spectrum of the " flash " 

 becomes at once apparent. Upon its characteristics and 

 upon their interpretation stand or fall our whole con- 

 ceptions of the chemical constitution of the sun. For the 

 " flash" is the revelation of the spectrum of the base of 

 the sun's atmosphere within the limits of the powers of 

 our present instruments. A depth of seven hundred 

 miles is an enormous one in any atmosphere, and especially 

 in that of the sun, and must include a vast range of 

 conditions, both of pressure and temperature ; but we 

 are at present compelled to treat it as an indivisible 

 integer. Keeping this fact in view, that the seven 

 hundred miles of depth of the " flash " stratum must 

 include a great number of very distinct minor strata 

 of which only the lowest can, on the old hypothesis, be in 

 complete correspondence with the Fraunhofer spectrum, it 

 is clear that we can test the rival claims by watching 

 whether or no, as totaUty comes on, the ever-increasing 

 bright horns which appear above and below the con- 

 tinuous spectrum are the reversals of the dark Fraimhofer 

 arcs. On the old hypothesis, the multiplying bright lines 

 should ever be approaching complete correspondence with 

 the Fraunhofer spectrum up to the moment of commence- 

 ment of full totality ; on Lockyer's hypothesis, they should 

 ever be diverging further from it. The conditions of 

 observation preclude us at present from following out the 

 process to its minutest and final detail. All we can do — 

 and it is sutt'cient — is to mark in which direction the 

 tendency lies. 



It is this question of the direction of progress which is 

 the crucial one — whether, as we get nearer the base of the 

 solar atmosphere, the bright-line spectrum becomes more 

 and more, or less and less, accordant with the Fraunhofer 

 spectrum. It is not a question of establishing a complete 

 and exact correspondence. That we could not expect. 



Nor is it a question of the relative intensities of the 

 lines. With that question we are not 3^et competent 

 to deal. It has been generally assumed (Sir Norman 

 Lockyer asserts it nakedly ) that the relative intensity of 

 the bright lines of the spectrum of any element in the 

 laboratory ought to be the same as that of those same lines 

 when dark in the Fraunhofer spectrum. Dr. .Johnstone 

 Stoney has recently reminded us how wholly unwarranted 

 this assumption is ; for if, as he puts it, we observe the 

 spectrum of some source of white light through a sodium 

 flame, and therefore see the D lines dark in a continuous 

 spectrum, and then increase the brilliance of the sodium 

 flame, we diminish the intensity of those dark lines. 



Dr. Stoney also points out that a diflerence of intensity 

 between the bright-line and the dark-line spectrum may 

 be due to the gas being present in but very small quantities. 

 Thus the D.j line of helium is very brilliant as a bright 

 line in the chromosphere, but is normally absent as a dark 

 line from the spectrum of the disc. We cannot tell cer- 

 tainly whether this is due to the helium being so bright 

 as to emit as much light as it absorbs from the sun, or 

 whether it is so tenuous as practically to absorb nothing 

 when we look at the sun through it, and only reveals itself 

 at the limb in consequence of the vastly greater depth we 

 look through ; or a combination of the two factors may 

 supply the complete explanation. For myself, believing 

 as I do that any true solar atmosphere must be limited to a 

 very few miles above the photosphere, and that chromo- 

 sphere and prominences, however magnificent in appear- 

 ance, are of the last degree of tenuity, I am disposed to put 

 much stress upon the second suggestion. The phenomena 

 of comets' tails might remind us how brilliant and far 

 reaching a body may be without any real substance. 

 Indeed, the corona itself is a case in point. We look down 

 upon the sun day by day through millions of miles of 

 depth of its strange, complicated structure, and are not 

 able to recognize the faintest sign of its presence. 



The " flash " past, the nest stage of the phenomenon is 

 one in which the prismatic camera still asserts its pre- 

 eminent usefulness. The corona, prominences, and chromo- 

 sphere, so far as these still lie outside the dark disc of the 

 moon, are now our source of light. The two latter give 

 us a bright-line spectrum only. The corona gives us a 

 bright-line spectrum plus a faint continuous one. We have, 

 then, still a number of bright arcs of different lengths in 

 the spectrum, and of different shapes ; for there is no 

 prominence, there is no elevation of the chromosphere, 

 however small, that does not give its own separate spectrum. 

 We find the counterfeit presentment of each painted over 

 and over again in each several tint that the lines of the 

 gases which compose it yield. One such photograph, 

 therefore, supplies us not with one spectrum, but with 

 many ; not with one representation of the chromosphere, 

 but many. Thus in Mr. Evershed's beautiful photographs, 

 taken during the total phase and reproduced in the June 

 Number, there is no point of light that is not significant, 

 no dot or line that has not its story to tell. 



I trust that I have in the foregoing paper succeeded in 

 impressing upon my readers some of the advantages of 

 the prismatic camera. A further advantage is that by its 

 extreme simplicity it is most economical of light. It is 

 not, indeed, theoretically a suitable instrument for the 

 determination of wave-lengths. Practically, so many of 

 the lines seen in an eclipse being thoroughly well known, 



* " The line least intense in the photograph ought to be the least 

 intense in Thalen's tables, and if it existed in the sun at all it ought 

 to be the least intense among the Fraunhofer lines." — "• Chemistrv of 

 the Sun," p. 231. 



