THE SUM'S CORONA AND HIS SPOTS. 25 



glass prisms of sixty degrees each, evidence, though slight 

 perhaps, might be obtained of the presence of the substance 

 which produces the green line. Thus variations in the con- 

 dition of the corona might be recognised, and any law 

 affecting such variations might be detected. I must confess, 

 however, that a consideration of the optical relations in- 

 volved in the problem leads me to regard the attempt to 

 recognise any traces of the corona when the sun is not 

 eclipsed as almost hopeless. 



It is clear that until some method for thus observing the 

 corona has been devised, future eclipse observations will 

 acquire a new interest from the light which they may throw 

 on the coronal variations, and their possible association in 

 some way, not as yet detected, with the sun-spot period. 

 Even when a method has been devised for observing the 

 gaseous corona, the corona whose light comes either directly 

 or by reflection from solid or liquid matter will still remain 

 undiscernible save only during total eclipses of the sun. 

 Many years must doubtless pass, then, before the relation 

 of the corona to the prominences and the sun-spots shall 

 be fully recognised. But there can be no question that the 

 solution of this problem will be well worth waiting for, even 

 though it should not lead up (as it most probably will) to the 

 solution of the mystery of the periodic changes which affect 

 the surface of the sun. 



