8 9 



and the course of research in any field can never 

 be clearly foreseen. In the study of the sun there 

 is much to be done in continuation and extension 

 of present work: the development, on an observa- 

 tional and experimental basis, of a theory of sun- 

 spots; the determination of the exact position of 

 the sun's magnetic poles and their period of revo- 



FIG. 64. Snow on Mount Wilson. 



lution about the poles of rotation; the extended 

 investigation of the electric and magnetic phe- 

 nomena, and the pressures and motions of gases 

 at different levels in the solar atmosphere. Stellar 

 problems are innumerable, but the work already 

 begun demands first consideration. We must 

 learn beyond doubt whether light is scattered in 



