xxiv.] INDIVIDUALITY OF LINES. 



lines. At a later date we have the line at 5382 at. rest, while 

 that at 5378 is in motion. 



These observations, then, prove there is just as much in- 

 dividuality in the way in which the lines of iron change their 

 refrangibility as there is in the way in which one particular 

 line, and then another, is thickened in a sun-spot or brightened 

 in a prominence ; and if we go further we find this very "inter- 

 esting and additional fact, that the lines which are not con- 

 torted are in a great many cases precisely those lines which 

 are seen in the prominences, but not in the spots. 



It is seen, therefore, that the evidence afforded by change 

 of refrangibility is of like nature to that afforded by the thick- 

 ening of lines in spots and brightening of lines in flames. 



The explanation which lies on the surface is that the vapours 

 in the flames produce one set of lines in one place or at a 

 certain temperature, and the vapours in the spots produce 

 another. Sometimes these vapours are mixed up by up-or- 

 down rushes, and sometimes, therefore, the lines are common. 



The diagram (Fig. 116) gives the main results in a convenient 

 manner. It does not profess to go over the whole ground, but 

 I think it will enable me to point out the way in which the 

 phenomena observed on the sun are re-echoed and endorsed by 

 the work which has been done in the laboratory, and how severe 

 the .tests applied have been, and how well the view has borne 

 the -strain. I have included in the diagram all the variations of 

 intensity referred to in the present chapter concerning the lines. 



The diagram refers to three iron lines visible in the map given 

 on page 339 three lines that in an instrument of ordinary 

 dispersion might easily be mistaken for a single line in the solar 

 spectrum. We have, as before, the intensities among the 

 Fraunhofer lines recorded in the upper part of the diagram ; we 

 then go to the photographs of the arc, and find that the line at 

 4923*2 is entirely absent. We then pass on to the quantity coil, 

 which gives us the three lines ; but there is a difference between 



