August 18, 1 881] 



NATURE 



367 



the sun, are really thin lines at the temperature of the electric 

 arc, but that they kept intensifying and were rendered visible 

 almost alone, when, instead of using the electric arc we used 

 an induced current of considerable tension. But when we 

 pass from the case of calcium, which occupied the attention of 

 solar observers several years ago, to other elements, and when 

 we go still more into the minute anatomy of the thing, we find 

 that the further we go the less final is the statement that the 

 matching in intensity of the lines is perfect. 



Nor is this all. A^ot only is the matching less perfect in 

 intensity^ but lohole reaches of lines in various spectra are left 

 out which cannot be accounted for on the long and short prin- 

 ciple. It has been before pointed out that of tlie 26 lines of 

 aluminium, 2 only being left in the solar spectrum is easily ex- 

 plained, because the 24 dropped were short lines. But when we 

 come to other elements, we find of adjacent lines — lines of equal 

 length, and which, so far as we can. gather, ought to be equally 

 represented in the sun — one is absent, and one is present, 

 probably with more intensity than it would seem to deserve from 

 its behaviour among other lines of the spectrum. A table will 

 best exhibit the sort of variation that crops up and insists on 

 being recorded when the solar spectrum is photographed in any- 

 thing like the detail which it absolutely demands. The method 

 of recording will be at once understood. 



Metal. 



Intensity in Intensity in Intensity, 

 sun. photograph. Thalen. 



i=darkest. i=brightest. i=brightest. 

 425 

 323 



Wave- 

 length. 



Fe / 4197-5 I 2 



\4i9S'i 3 2 - Stronger line. 



Co i4"'8o 2 I 



\4I20'5 4 I - Stronger line 



Ni / 4458-6 

 N' 1 4647-8 



Tr / 4344-4 4 3 



^' 143SI-S 3 3 



Mo 1 4706-5 

 U757-5 



w -f 4842-0 



I48S7-5 



Ti / 3980-8 2 I 



13989-25 I I 



In / 4679-5 3 « I 



14721-4 4 I I 



p (4442-0 424 



\45S"-8 3 2 2 



Pd {3893-0 I I 



1 3958-0 3 I - Stronger line. 



Zr / 3957-22 2 i 



1 3990-45 3 I - Stronger line. 



Di (3989-65 o I 



1,3993-98 3 I 



ji, /42oro 012 Stronger line. 



1 4215-5 3 I o 



Now if Kirchhoff's view be anything like a representation of 

 the whole truth there ought not to be any difference between 

 these intensities ; the line least intense in the photograph ought 

 to be least intense in Thalen's tables, and if it existed in the 

 sun at all, it ought to be least intense amongst the Fraunhoferic 

 lines, but as a matter of fact, there is an absolute inversion. 

 The cobalt line 4120-5 is four times as intense in the sun as 

 in the photograph; in the titanium line 39S9-25 the intensities 

 are equal ; while in tungsten 4S42-0 they are inverted, being 

 represented as of minimum intensity in the sun, and maximum 

 by Thalen and in the photograph. In the sun one of the lines of 

 iron is given a.s of first, and the other as of third intensity, while 

 in the photograph they are both of second order. Again, in 

 didymium we get a first order line recorded in the photograph 

 which is absent from the sun altogether, whereas another line of 

 the first order near it is there as a line of small intensity ; so also 

 in rubidium, and so we might go on. Indeed it is evident that 

 the moment we go into minute details in this work we find that 

 the general statement requires a very considerable amount of 

 modification. And in addition to that too, there comes the ques- 



tion, how on this theory of the identity of the nature of the 

 substances in the earth and the sun, are we to account for the 

 bright lines seen in the sun itself— for the bright lines seen in 

 the photosphere, to say nothing of those seen in the chromo- 

 sphere — which have no corresponding Fraunhofer lines at 

 all — lines so numerous that in a prominence of moderate 

 complexity we may say that half the lines are absolutely un- 

 known to us? Now when the other lines observable under 

 these conditions — lines which we can get accurately, are lines 

 known to us (we are dealing « ith the product of the very highest 

 temperatures which we can command) we are justified, I think, 

 in imagining that these lines which we do not get at, are lines 

 which we could get at if we could proceed a little further. They 

 elude our grasp ; we know nothing about them ; we put a query 

 against them all because we cannot get at that stage of tempera- 

 ture at which they are produced. 



There is one very beautiful case of this kind that comes 

 out from Tacchini's observations (Fig. 39). From the beginning 

 of Februarj', 1872, Tacchini had observed the two iron lines 

 4922-5, 5016-5, when suddenly the whole rhythm of his 

 observation was broken, and at the end of December, 1872, 

 these iron lines ceased to be visible in the flames altogether. 



On no one occasion after this for some time was either of 

 these iron lines observable, but from January to September, 

 1873, he saw two lines of wave-lengths, 4943 and 5031, about 

 which absolutely nothing whatever is known ; so that it really is, 

 I think, a perfectly justifiable suggestion that these lines are the 

 spectrum of a substance which exists in the flames which is pro- 

 duced at a much higher temperature than that needed to give us 

 those other forms of " iron " which produce the lines in the spots. 



That is a suggestion which is obvious from a reference to 

 the maps, and if it is correct we must acknowledge that when 

 the sun was in that intense state of quiescence that there were no 

 downward currents— nothing to bring the cooler vapours from 

 the higher regions of the sun down to obstruct the general tenour 

 of the solar way in the flame region, that at last, in consequence 

 of this wonderful tranquillity, even the iron lines — the only two 

 lines which indicate the presence of iron in the flames — faded 

 away because iron, as we know it, faded away. There is no 

 other explanation that I know of. In .addition to those two 

 lines we have two other lines about which we know nothing, 

 except that they are probably due to a temperature which we 

 cannot approach. 



Special Test with regard to Iron 



Part of the work which has been undertaken in connection 

 wMth this special branch of the investigation!, has been a careful 

 inquiry into the changes brought about in the spectrum of iron 

 by exposing it to as widely different temperatures as possible. 

 The research is a very laborious one, and it may be some day 

 we shall get a very much better record than that which my 

 assistants and myself have produced ; what we have been able to 

 do we have done over the region of tlie spectrum which we have 

 already worked over in the spots and flames. 



In difi-erent horizons we have recorded the results observed 

 when we use either the arc or the coil, or the oxyhydrogen flame 

 or the Bessemer flame or some other liglit-source, and we vary 

 in each case, as far as can be, the temijerature employed. For 

 instance, when we use the quantity coil we use a big jar, a little 

 jar, and no jar at all ; and the same with the intensity coil. Now 

 if this map is carefully studied,' it will be seen that the inten- 

 sity of the lines is very considerably changed when we pass 

 from one ^et of observations taken under one set of conditions, 

 to another set taken under other conditions. It is not a mere 

 question of dropping out the lines when we pass from the tem- 

 perature of the arc to the temperature of the coil, but it really 

 is a considerable intensification of certain of the lines tinder 

 certain conditions. There are three conditions under which 

 we get the \.\\o lines 5339 ^fid 5340> ^nd 'h^y ^""e 110' seen after- 

 wards. The line 5433 is seen ratlier faint in the sun and very 

 strong in the Bessemer flame. 5197-5 is very faint in the sun, 

 but its intensity is doubled and even trebled with certain con- 

 ditions of the quantity coil. I have introduced these facts to 

 point a remark about Kirchhoff's statement ; when Kirchhofif 

 made that statement he was amply justified by the science of the 

 time. He was familiar naturally with the spectrum of iron, 

 which he had studied in his own laboratory, and other good 

 observations of the spectrum of iron had been recorded in 

 his time. Bui, with observations like these before one, which 



^ The map is too large and too detailed to be reproduced here. 



