536 THE POPULAR SCIENCE MONTHLY. 



that (if we rely on the correctness of the observations "ust alluded to) 

 the average brilliancy of each of these bodies cannot be much less 

 than five times that of full sunlight, " sunlight " itself being, in fact, 

 caused by a dilution of their brilliance with that of the gray back- 

 ground which has been just compared to the cloth on which white 

 grains are grouped. 



Tliere are inequalities in the brightness of these bodies, some of 

 which fall below, while others as certainly exceed, the average. If we 

 remember that each of them occupies an area larger than Great Brit- 

 ain, that this area is, throughout, brighter than the sun (in fact, not 

 in metaphor), and that such enormous bodies, whatever they may be, 

 exist in the sun in numbers which are almost incalculable, we reach up 

 to some idea, but doubtless an inadequate one, of the incomprehensi- 

 ble vastness of the solar sphere, and of the interest of the problems it 

 offers for our study. 



In order to examine these bodies under other conditions, we must 

 look at a sun-spot. Here, again, we find it difficult to conceive the 

 vastness of the field of operations, for, including both branches, 

 the " spot " rejjresented. in our engraving covers over 1,000,000,000 

 square miles. If we fail utterly to " realize " the extent this repre- 

 sents, we may, perhaps, derive aid from its comparison with some 

 familiar terrestrial object. In the small circle, accordingly, the conti- 

 nents of North and South America have been drawn on the same 

 scale as the spot, as they would appear ; that is, if they were actually 

 transported to the solar surface, placed beside the spot, and viewed, 

 together with it, from the distance at which the earth is from the sun. 



The engraving is from a drawing by the writer. This drawing, 

 while representing the general outline of a particular spot, seen in 

 March, 1873, embodies the result of many previous studies on similar 

 ones, and it has been made much less with an attempt to gain pictorial 

 effect than to truthfully present such features as will help to give some 

 idea of the constitution of the solar surface. 



We see that each branch of the spot consists of two main parts, an 

 outer (the penumhra) and an inner (the umbra), and beyond this rude 

 division little seems to have been observed till recent years. The 

 knowledge of the real complexity of spot-structure and the fullness of 

 detail needed to represent it are of such recent origin that Sir John 

 Herschel, who, in the Cape-of-Good-Hope observations, has given a 

 number of sun-spot drawings, points out, in one of them, the tendency 

 to a radial structure, as something remarkable and nearly unnoticed ; 

 and the fact that so eminent an observer should have made the spots 

 his careful study without detecting more of the structure since dis- 

 covered, will illustrate the difficulties attendant on such an investiga- 

 tion. If we look at this, not merely as at a picture, but in the way in 

 which we should examine a geological map, with the purpose, that is, 



