March 25, 1886] 



NATURE 



499 



THE SUN AND STARS'' 



IV. 



Changes of Waue-Length 



"T^HE spectroscope not only enables us to determine the chemical 

 ■*■ constitution of the spots, but it allows us, by the alteration 

 in the refrangibility in the various lines, to determine the rates of 

 motion at which each of the vapours is descending. The hydro- 

 gen lines are not thickened as the other lines are in the spots ; 

 they are at times thickened on one side only, that is to say, they 

 are what is called contorted. The line suddenly changes its 

 position in the spectrum towards the less refrangible part of the 

 spectrum, which is the red end. Now the amount of change 

 towards the red along the spectrum measures exactly for us the 

 rate of downrush, and we learn generally that the hydrogen is 

 rushing down at the rate of 30 or 40 miles a second. These 

 changes are associated with brightenings of the line, which we 

 shall have to refer to afterwards. 



The fact that we get those extraordinary inversions above 

 referred to of the various lines in the spectrum of a substance is 

 simply explained by the assumption that the substance is an 

 exceedingly complicated thing that is broken up into simpler 

 things at the temperature of the sun, and that some of these 

 things exist in some sunspots, while other constituents exist 

 in others. The changes of wave-length come to the support 

 of this argument. If a chemical element is one thing, it cannot 

 be doing two different things at the same time ; if its vapour in 

 a spot is homogeneous, one set of lines cannot tell us that it is 

 going up and another that it is going down ; but that is exactly 

 what the sun does tell us. 



Those results, like the previous ones relating to the inver- 

 sions, of course are simply and sufficiently explained by the 

 assumption that we are not dealing in any one vapour with one 

 set of molecules only, but that we are really dealing with various 

 constituent molecules, and that some of the molecules may be in 

 one stage of simplicity, some of them may be in another; some 

 may be at rest, while the others may be in very violent motion. 

 These observations of the relative movements of the vapours 

 have been made at different times and in different parts of the 

 spectrum. We get, in all of them, contorted lines, showing us 

 that a particular vapour is moving ; while other lin»s in the 

 spectrum of the same substance indicate that there is no move- 

 ment whatever in the vapour in that particular spot. 



This problem, however, has a very great difficulty connected 

 with it, because it will be readily understood that the slit of the 

 spectroscope has to be kept absolutely on the same part of the 

 sun. It would not do, it would not be fair, for instance, to have 

 the slit of the spectroscope resting on one part of the sun, and 

 then make an observation of a line of any particular substance 

 indicating motion, and then to let it, even by accident, travel on 

 to another part of the spot and find out that the next line of the 

 same substance indicates rest. The answer to that would be — 

 You are dealing with two different parts of the spot. The 

 observations must be made contemporaneously. I may tell you 

 parenthetically that we have a new instrument now which I think 

 will help us very considerably in these inquiries. This is a 

 spectroscope having a diffraction-grating with 17,000 lines to 

 the inch, which gives us, therefore, a very considerable disper- 

 sion. What has been done has been to cut the grating in two. 

 If the two parts of the grating are absolutely in the same plane, 

 of course the whole grating will be receiving the light which 

 comes from the sun direct into the spectroscope, and will send a 

 definite part of the spectrum to the eye ; both parts of the 

 grating then will be building up a spectrum of the same part of 

 the sun, and will give us the result in the same part of the spec- 

 trum. But now that we have split the grating we have the 

 power of altering the inclination of one half of it, while the 

 other half remains rigidly in its first position. Hence we can 

 make any part of the spectrum overlap any other part, and in 

 this way, instead of being limited to observations of parts of 

 the spectrum sufficiently close to each other to be visible in the 

 same field of view, we can compare lines in the red part of the 

 spectrum thrown to the eye by one part of the grating with 

 lines in the blue part of the spectrum thrown by the other. In 

 this way we are able to make comparisons from one end of the 

 spectrum to the other, when there is no doubt whatever that the 



^ A Course of Lectures to Working Men delivered by J. Norman Lockyer, 

 F.R.S., at the Museum of Practical Geology. Revised from shorthantl 

 notes. Continued from p. 472. 



slit is lying on the same part of the spot ; this will be an enorinous 

 safeguard against error. 



We have taken several photographs of the spectra of sun- 

 spots at Kensington ; nearly all of them indicate that two lines of 

 calcium in the ultra-violet spectrum — two lines it is difficult to 

 see with the eye — are always bright, while all the rest remain 

 dark. Another important fact is that in addition to the down- 

 rush, the velocity of which has been already stated as some- 

 thing like 40 miles a second on the average ; there are in the 

 neighbourhood of the spots, in consequence of the disturbances 

 produced, violent movements of the lower parts of the solar 

 atmosphere which we should call winds ; that is to say, in- 

 stead of being up and down they are really horizontal along the 

 surface of the sun. Now 140 miles a second is no uncommon 

 velocity for these winds, and we may imagine, therefore, that 

 the heated gases of the photosphere, and the cooler gases of the 

 spots, are very often arranged in layers. When this is so it is 

 easy by the appearance of the widened absorption-lines to 

 determine the existence of a hotter layer above or between cooler 

 ones. We get what is called a double reversal of the lines. 



Fig. 8. — Spectrum showing the double reversal of the _D lines due to hotter 

 sodium vapour overlying cooler sodium vapour in the sunspot. 



Prof. Young, of America, has been fortunate enough to 

 detect one case with, I think, no less than three strata of incan- 

 descent sodium vapour inter-layered between three strata of 

 relatively cool sodium vapour. When we wish to consider the 

 phenomena of spots in their entirety therefore, we must not con- 

 sider the downrush on the photosphere merely, but must also bear 

 in mind the lateral currents which are produced by the disturb- 

 ances it has set going. 



Conchtsions 

 I will now state the conclusions at which we have arrived 

 touching sunspots by means of the work which I have 

 brought before you. You will see they are all of them im- 

 portant with regard to the structure of the lower part of the solar 

 atmosphere. 



(1) The spot spectra are very unlike the ordinary Fraunhofer 

 spectrum. 



(2) We get as much inversion of lines in the case of one ele- 

 ment as we do between the lines of different elements ; by which 

 I mean that the lines of nickel, say, are just as much varied in 

 different spots as the lines of iron, nickel, and calcium would be 

 in spots in which the proportions of these substances very greatly 

 varied. 



(3) Very few lines indeed are strongly affected at the same 

 time. A great many lines of the same substance are affected of 

 course besides those included in the twelve which have been re- 

 corded at South Kensington as most widened every day ; but a 

 small number of the lines altogether are affected in this manner. 



(4) There is a change depending on what I shall afterwards 

 have to refer to as the sunspot period ; that is to say, day after 

 day, month after month, year after year, the lines of any par- 

 ticular substance thickened in the spots are not the same. In 

 fact towards the end of the year 1881 (we began our observations 

 in the year 1S79) the iron lines which were strongly affected at 

 first died out altogether. 



(5) Many of the lines seen in the spots are lines seen at low 

 temperatures, and none of them are brightened or intensified 

 when we pass from the temperature of the electric arc to that of 

 the electric spark. 



(6) In the first 200 spots observed loi lines were recorded 

 which have never been mapped in any laboratory ; that is to say, 

 they do not correspond with lines seen in the emission-spectrum 

 of any substance with which we are familiar. 



(7) Many of the lines widened are new solar lines ; that is to 

 say, they are not visible among the Fraunhofer lines at all. 



