August 4, 1881] 



NATURE 



317 



we have independent evidence that the star with very few lines is 

 hotter than our sun. It is also clear that the other star with the 

 fluted spectrum is a star much cooler than our sun, because it 

 was one of those red stars, the light of which is exceedingly 

 feeble, which, on grounds independent altogether of spectroscopic 

 evidence, are supposed to be stars in the last stage of visible 

 cooling. ' 



So much then for some of the earlier observations on the coin- 

 cidence of metallic lines in the sun, with observations on the 

 lines themselves in different portions of the sun's atmosphere. 



10. We now come to another part of the work where we also 

 find difficulties. Angstrom, in that exceedingly important memoir 



which accompanies his Atlas, states: ' "In increasing successively 

 the temperature I have found that the lines of the spectra vary in 

 intensity in an exceedingly complicated way, and consequently 

 new lines even may present themselves if the temperature is 

 raised sufficiently high." Kirchhoff, on his part, had seen phe- 

 nomena very similar to those thus touched upon by Angstrom, 

 but his explanation was a different one. He did not agree that 

 the temperature upon which Angstrom laid such strong stress 

 was really the cause at work. He attributed those changes 

 rather to the mass and the thickness of the vapours experimented 

 upon — nay, he went further : at a time when scarcely any facts 

 were at his command he broached a famous theorem which went 



Fig. 28. — 1 he blue end of the spectrum of calcium underdiftereut conditions, j. Calcium combined with chlorine (CaCia). When the temperature is low, 

 the comptund molecule vibrates as a whole, the spectrum is at the red end. and no lines of calcium are seen. 2. The line of the metal seen when 

 the compound molecule is dissociated to a slight extent with an induced current. 3. 'I he spectrum of metallic calcium in the electric arc with a small 

 number of cells. 4. The same when the number of cells is increased. 5. The spectrum when a Gotland small jar are employed. 6. The spectrum 

 when a large coil and large jar are used. 7. The absorption of the calcium vapour in the sun. 



to prove this ; and yet what had Kirchhoff himself done ? how had 

 he traversed his own theory ? He states that his observations 

 were made by means of a coil using iron poles one millimetre in 

 thickness. Now the thickness of a short spark taken from iron 

 poles one millimetre in thickness would probably be two milli- 

 metres. Next Kirchhoff allocated the region where the absorp- 

 tim which produces the reversal of the iron lines took place at a 

 considerable height in the atmosphere of the sun, and he 

 expected the atmosphere of the sun to be an enormous mass 

 represented by the old drawings of coronas, so that on Kirchhoff 's 

 view the thickness of the iron vapour which reversed the iron 



sp«frum must have been, at a moderate estimate, 10,000 miles, 

 and yet he said that the spectrum of that, and of the light given 

 by the coil were absolutely identical ; that is to say, that the fact 

 was that the variation of thickne-s from two millimetres to 

 10,000 miles made no difference. That was on the one hand ; on 

 the other hand he gave us his theorem, showing that a slight 

 variation of thickness would produce all the changes which 

 Angstrom and others had observed up to that time, and which 

 we have observed since in much greater nu 1 ber. 

 . A diagram (Fig. 28) will show the sort of changes to which 

 Angstrom referred, ch:in;;es «hicli have been observed by every 



new worker who has taken up the subject. It represents 

 the variations which take place in the spectrum of calcium in 

 the photographic region. At a particular temperature we get 

 a spectrum of calcium which contains no lines whatever in the 

 blue, but when we increase that temperature — the temperature 

 of a Bunsen burner is sometimes sufficient to produce it — we get 

 a line in the blue. When we pass from a Bunsen burner to an 

 electric lamp we get this blue line intensified and reversed, and 

 at the same time we get two neiv lines in the violet. Using a 

 still higher temperature in the arc, we thin the blue line, and at 

 the expense of that line, so to speak, we thicken the two in the 



; ectmm of iodine- 

 violet, so that the latter equal the blae line in thickness and 

 intensity. Passing to a large induction coil with a small jar we 

 make the violet lines very much more prominent, and usmg a 

 larger induction coil and the largest jar we can get, wc practicaUy 

 aboUsh the blue line and get the violet lines alone. Now we 

 have sinipiv produced these elTects by varying the temperature, 

 and this diagram enables me to point out one of the thing.s to 

 which reference w ill have to be made subsequently. The thick- 

 nesses of the calcium lines in the spectrum of the sun are also 

 given. The two lines in the violet are really H and K. ine 

 ' " Recherches sur le SpecUe Solaire," pp. 38, 39- 



