270 



NA TURE 



\yuly 21, 1881 



the presence of zinc in the sun remains doubtful. It is note- 

 worthy, liowever, tliat there are three lines in the magnesium 

 spectrum which present the same nebulous appearance, and to 

 which there are no corresponding solar lines, and yet mag- 

 nesium is undoubtedly present in the sun. 



Kirchhoff's and Angstrom's maps are in all our labora- 

 torie^, and there is a very considerable difference between tlieiii. 

 This difference arises from the fact that whereas Kirchhoff used 

 an induction coil and spark. Angstrom varied his experimental 

 method by placing no longer a spark, but the electric arc in fiont 

 of the slit of his instrument. In this case, therefore, he was 

 determining the spectrum which was produced at the temperature 

 of the electric arc instead of the spectrum which was produced 

 at the temperature of the induction coil. The result of their 

 combined attack is shown in the accompanying table : — 



Elements present in the Sun 

 Kirchhoff. Angstrom and Thalen. 



Sodium. Sodium. 



Iron. Iron. 



Calcium. Calcium. 



Magnesium. Magnesium. 



Nickel. Nickel. 



Barium. 

 Copper. 

 Zinc. 



Chromium. 

 Cobalt. 

 Hydrogen. 

 Manganese. 

 I Titanium. 



So far then for that mode of observing the sun which consists 

 in comparing the total light of the light-source with the total 

 light of the sun. 



This introduces an important consideration. When we have a 

 light source placed in front of the slit of the spectrorcope it is per- 



FlG 7 — The eyepii 



wuh spectrjscope attiched. 



fectly clear that light from all portions of the light source nuHt 

 illuminate the slit. Similarly, if we content ourselves by 

 pointing the spectroscope to the sun, or to a cloud illumi- 

 nated by the sun, it is perfectly obvious that the light from 

 all parts of the sun must enter all parts of the slit. 



Is there any other way of observing the sun along with the light 

 source? You will see in a moment that there is. JVe can t/iiow 

 an image of the sun on the sUt of the speclrosope. This w ork 

 was begun in lS66. If an image of the sun contains, let us say, 

 a spot or a f.acula, we can see it w hen we throw it on to the slit. 

 If we can manage to do so we shall get the spectrum of the sun- 



spot as distinguished from the spectrum of the other portions of 

 the sun, or we shall get the spectrum of the facula as ojjposed to 

 the spectrum of the other portions of the sun. The manner in 

 which this kind of work is carried on is easily grasped. It 

 simjily consists in the use of a spectroscope of large dis- 

 persion attached at the focal point of a telescope of considerable 

 power. Here is the eye-piece end of Mr. Newall's refractor, 

 with a spectroscope, with a considerable number of prisms, fixed 

 to the telescope by means of an iron bar, with the slit of it in 

 the position of the focus, so that when the instrument is pointed 

 towards the suu we see an image, in the case of this telescope 

 something like four inches in diameter, with the spots and 

 brighter portions wonderfully and beautifully clear, and by 

 means of the different adjustments of the telescope we can 

 bring now a spot, and now one of the brighter portions of the 

 sun on to the slit, and see if there be any difference between the 

 spectrum of the spot and the spectrum of the general surface of 

 the sun. 



If we wish to observe two adjacent spots and compare their 

 spectra, we can rotate the spectroscope and look at both. 

 Again, anticipating matters, I can show what we see to a 

 certain extent, for latterly we have been fortunate enough to 

 obtain s jme photographs of the spectra of sun-spots. 



Fic. S.— Spectrum of Sun-spot, showing the widening of the D lines. 



The dark portion gives us the spectrum of the spot throughout 

 the whole length of the spectrum. That is a case of continuous 

 absorption. The continuous radiation of the sun is cut off, but 

 independently of this continuous absorption some of the lines are 

 considerably thickened in the nucleus of the spot (Fig. 8). Now 

 the lines observed in the first instance were the lines of sodium, 

 and the point of the observation was this. Two rival theories 

 had been suggested to explain how it was that the sun-spot was 

 dark. One school said it was due to absorption, and another 

 that it was due to the defect of radiation from the interior gases 

 of the sun. If we had been deaUng with defective radia- 

 tion, we should still have been dealing with radiation, and 

 should have expected to see bright lines ; but no obvious 

 bright lines were seen in the spectrum of the spot ; what we 

 did see was the thickening and darkening of the lines and the 

 continuous absorption. In the case of the lines of sodium it 

 was very marked ; so that we were perfectly justified in saying that 

 the sun-spot was really not produced by any defect of radiation, 

 but was truly and really produced by an increased amount of 

 absorption. 



I hope to show you that we can vary the thickness 01 

 this line in precisely the same way that it is varied in the dif- 

 ferent sun-spots, and if then we examine the conditions under 

 which we can experimentally make the line thicker, we shall in 

 that way get some explanation of the thickening of the line in 

 the solar spot. This experiment is rather a diflicult one. We 

 will volatilise some sodium in the electric arc and throw its 

 spectrum on the screen. I hope to show that the absorption 

 line is very thick to start with, and then it becomes very thin ; 

 if I give it time it will thin down gradually. What is the cause 

 of the thickening and the thinning? It is perfectly obvious. 

 The temperature is practically the same all the time, but we 

 ha\e a very considerable quantity of sodium vapour surrounding 

 the incandescent p lies in the first instance. On the further ap- 

 plication of the heat this sodium vapour goes away by degrees, 

 and we gradually deal w ith a smaller quantity, and as we deal 

 with a smaller quantity the line thins. We therefore are 

 justified in -aying that when in a sun-.spot we get the line of 

 sodium considerably thickened, that is due to the fact that in a 

 sun-spot there is a greater quantity of sodium vapour present. 



