Page:Popular Science Monthly Volume 5.djvu/554

536 that (if we rely on the correctness of the observations just 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 background which has been just compared to the cloth on which white grains are grouped.

There 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 Britain, 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 incomprehensible 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" represented in our engraving covers over 1,000,000,000 square miles. If we fail utterly to "realize" the extent this represents, we may, perhaps, derive aid from its comparison with some familiar terrestrial object. In the small circle, accordingly, the continents 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 penumbra) 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 discovered, will illustrate the difficulties attendant on such an investigation. 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,