Smithsonian Report, 1931. — Russell 



Plate 2 



A Great Sun Spot. Enlarged From a Direct photograph (Mount Wilson 



Observatory) 



It will be noticed that the whole solar surface shows an indistinctly mottled appearance. There are no 

 clouds on the sun. This mottling is due merely to temperature ditlorences which cause ditlerences of 

 brightness of the hot bases which send us light. 



