April 20, 1899] 



NA TURE 



587 



making the comparisons which were necessary for 

 continuing the inquiry. 



The recent work has been done with this object in 

 view. 



The way in which the enhanced lines have been used 

 is as follows. Those belonging to some of the chief 

 metallic elements have been brought together, and thus 

 form what I have termed a " test-spectrum." This has 

 been treated as if it were the spectrum of an unknown 

 element, and it has been compared with the various 

 spectra presented by the sun and stars. 



How marvellous, how even magnificent, the results of 

 this inquiry have been, I shall show later in detail ; but 

 I may here say by way of anticipation that the test- 

 spectrum turns out to be practically the spectrum of the 

 chromosphere ; that is, the spectrum of the hottest part 

 of the sun that we can get at, and that a star has been 

 found in which it exists almost alone, nearly all the lines 

 of which had previously been regarded as "unknown." 



This last result is of the highest order of miportance 

 because it should carry conviction home to many who 

 were not satisfied with the change of spectrum as seen in 

 a laboratory, where, of course, the enhanced lines when 

 seen in the spectrum of the centre of the spark have along- 

 side them the lines in the spectrum of the outer envelope, 

 which of course is cooling, and in which the finer mole- 

 cules should reunite. For twenty years I have longed 

 for an incandescent bottle in which to store what the 

 centre of the spark produces. The stars have now 

 provided it, as I shall show. 



Although I have promised to pass over the history of 

 the work generally, 1 must still point out that the 

 enhanced lines in the test-spectrum actually include all 

 those first studied years ago when everything was dim, 

 and we were seeing through a glass darkly ; not as we 

 are now, face to face. To show the rigid connection of 

 the new with the old, it is desirable to refer briefly to 

 some of the work undertaken in relation to some of the 

 first anomalies noted. 



One advantage of this method of treatment is that it 

 shows that the immense mass of evidence now available 

 supports all the conclusions drawn from the meagre 

 evidence available a quarter of a century ago. 



Some of the anomalies were as follows : they are 

 given as specimens of many. 



(i) Inversion of intensity of lines seen under dfferent 

 circumstances. 



I showed in 1879 that there was no connection whatever 

 between the spectra of calcium, barium, iron, and man- 

 ganese and the chromosphere spectrum beyond certain 

 coincidences of wave-length. The long lines seen in 

 laboratory experiments are suppressed, and the feeble 

 lines exalted in the spectrum of the chromosphere. In 

 the Fraunhofer spectrum, the relative intensities of the 

 lines are quite different from those of coincident lines in 

 the chromosphere. 



(2) The simplification of the spectrum of a substance 

 at the temperature of the chromosphere. To take an 

 example, in the visible region of the spectrum, iron is 

 represented by nearly a thousand Fraunhofer lines ; in 

 the chromosphere it has only two representatives. 



(3) In sun-spots we deal with one set of iron lines, in 

 the chromosphere with another. 



(4) At the maximum sun-spot period the lines widened 

 in spot spectra are nearly all unknown ; at the minimum 

 they are chiefly due to iron and other familiar substances. 



(5) The up-rush or down-rush of the so-called iron 

 vapour in the sun is not registered equally by all the iron 

 lines, as it should be on the non-dissociation hypothesis. 

 Thus, as I first observed in iSSo, while motion is some- 

 times shown by the change of refrangibility of some lines 

 attributed to iron, other adjacent iron lines indicate a 

 state of absolute rest. 



Laboratory work without stint has been brought to 



NO. 1538, VOL. 59] 



bear, with a view of attempting to explain the anomalies- 

 to which attention has been directed. 



I only refer here to the work done on iron, magnesium- 

 and calcium, to show that in those metals the anomalies 

 were to a large extent due to the lines now termed 

 enhanced — that is, the lines seen to considerably change 

 their intensities when the highest temperatures are em- 

 ployed. 



I>-on. 



In the course of my early observations of the spectrum 

 of the chromosphere, I discovered on Jiine 6, iS6g, a 

 bright line at 1474 on Kirchhoffs scale, which I stated to 

 be coincident with a line of iron. On June 26 I dis- 

 covered another at 2003'4 of the same scale. 



The later researches on the spectrum of iron have 

 shown that the iron line which I observed in 1869 to be 

 coincident with the bright chromospheric line at 1474 

 on KirchhofPs scale, having a wave-length of 531679, is 

 an enhanced line, agreeing absolutely with Young's latest 

 determination of the wave-length of the 1474 chromo- 

 spheric line. 



Similarly the line at 2003'4 of Kirchhoff s scale, with a 

 wave-length of 4924, is also an enhanced line of iron. 



The first experiments were made to explain my own and 

 the Italian observations of the chromosphere which 

 proved the presence of only these two lines of iron in the 

 part of the spectrum ordinarily observed ; the ordinary 

 spectrum of iron in which 460 lines had been mapped 

 at that time was entirely invisible. 



The anomalies were investigated in the experimenf;al 

 work with sparks produced by quantity and intensity coils 

 with and without jars in the circuit. The outcome of 

 these experiments was to show that the chromospheric 

 representatives of iron were precisely the lines which 

 were brightened on passing from the arc to the spark, 

 while the lines widened in spots corresponded to a lower 

 temperature. 



The next anomaly observed was that in a sun-spot the 

 iron line at 4924 often indicated no movement of the iron 

 vapour, while the other iron lines showed that it was 

 moving with considerable velocity. 



It seemed perfectly clear then that in the sim "we 

 were not dealing with iron itself, but with primitive forms 

 of matter contained in iron, which are capable of with- 

 standing the high temperature of the sun, after the iron 

 observed as such, has been broken up, as suggested by 

 Brodie." ' 



On this view, the high temperature iron lines of the 

 chromosphere represent the vibrations of one set of mole- 

 cules, while the lines which are widened in spots cor- 

 respond to other molecular vibrations. Similarly, the 

 idea of different molecular groupings provides a satis- 

 factory explanation of the varying rates of movement of 

 iron vapour indicated by adjacent lines, the lines being 

 produced by absorption of different molecules at different 

 levels and at different temperatures. 

 Magnesium. 



In 1879 I passed a spark through a flame charged with 

 vapours of different substances. In the case of magnesium 

 the effect of the higher temperature of the spark was very 

 marked ; some of the flame lines being abolished, while 

 two new ones made their appearance, one of them at 

 4481. The important fact was that the fines special ta 

 the flame did not appear among the Fraunhofer lines, 

 while those of the spark did appear. 



This line at 44S1 now takes its place among the 

 enhanced lines like those of iron previously mentioned ; 

 special cases now form part of the more general one. 



Here again the experiments pointed to varying degrees 

 of dissociation at different temperatures as the cause of 

 the non-appearance of some of the magnesium lines irt 

 the Fraunhofer spectrum. 



1 Proc. Roy. Soc, vol. .\xxii. p. 204. 



