348 THE CHEMISTRY OF THE SUN. [CHAP. 



One word as to prominences. 



Some two or three years ago, when the sun-spot work 

 revealed the different behaviour in different spots of lines 

 visible in the spectra of the same element, it seemed desirable 

 to extend similar observations to metallic prominences, and, if 

 possible, in such a way that comparisons over a considerable 

 reach of spectrum should be possible. 



It then struck me that a grating cut in half, with one part 

 movable, would afford a ready means of doing this. Cir- 

 cumstances prevented the realisation of this scheme till 

 recently, when I put into the optician's hands a grating 

 presented to me by Mr. Rutherfurd. 



The result is excellent. It is possible to observe C and F, 

 for instance, together, quite conveniently, with either a normal 

 or a tangential slit. The only precautions necessary are to see 

 that half of the light passing through the object-glass falls 

 on the half grating, and that the rays which come to a focus 

 on the slit plate are those the wave-lengths of which are 

 half way between the wave-lengths of the two lines compared. 



It is to be regretted that the method will not work so well 

 with spots. The mixture of lines is too great, as two super- 

 imposed parts of spectra, both full of Fraunhofer lines, are 

 then in question. 



So far as this inquiry has gone at present we have observed 

 the lines contorted in spots only in the case of lines seen in the 

 same field of view at the same time. 



In the diagram (Fig. 115) the zigzag lines indicate the iron 

 lines which changed their refrangibility in a number of spots 

 observed at the end of 1880. The point is that, although we 

 have a great many of the iron lines bent, twisted, contorted 

 with their refrangibility changed, yet some of the iron lines 

 mixed with them give us no indication of movement. In 

 the diagram we have at 5366-70, three lines, two in motion 

 and one at rest, all belonging to a well-known group of iron 



