

xxiv.] CHANGE OF REFRANGIBILITY. 347 



5. The Test supplied by Change of Ref Tangibility. 



We have then got so far. Limiting our studies to iron we 

 find that the prominence spectrum is made up of one set of 

 lines seen in the terrestrial spectrum, and the spot spectrum is 

 made up of another set. And more than this, if we add the 

 lines seen in the prominence and spot spectra together we do 

 not then by any means make up the complete spectrum. 



Reference has already been made to another means of es- 

 tablishing this extraordinary fact of the separation of the iron 

 lines in spots and storms the change of refrangibility of the lines 

 brought about by the change of velocity of movement of the vari- 

 ous solar vapours. If, as already hinted, the lines of iron behave 

 to each other in precisely the same way as the lines of two 

 perfectly distinct substances behave to each other; then if we 

 observe changes of refrangibility in the iron lines, both in spots 

 and flames, we should get the same differentiation as we have 

 already got in the lines thickened or intensified in the spectra 

 of spots and flames. 



For such observations as these it is absolutely imperative 

 that the variations in the refrangibility of the lines, if they 

 exist, should be seen in the same field of view at the same time. 

 There must not be the least possibility of suggesting that the 

 variations are due to the observations being made on different 

 portions of the sun. 



We will now see the results which have been obtained along 

 this line of research, and it should be pointed out that it is not 

 a method by which it is easy in a short time to accumulate a 

 large number of observations, seeing that in the case of spots 

 we not only want a spot, but we want that spot to be in a 

 very considerable state of commotion, in order that the change 

 of refrangibility may be obvious enough to enable us to 

 record the phenomena. 



