Page:Hector Macpherson - Herschel (1919).djvu/43

Rh interior was visible. "Is it not reasonable," he asked, "to think that the great and stupendous body of the Sun is made up of two kinds of matter, very different in their qualities: that by far the greater part is solid and dark and that this immense and dark globe is encompassed with a thin covering of that resplendent substance from which the Sun would seem to derive the whole of his revivifying heat and energy?" Herschel's earliest observations were confirmatory of Wilson's conclusions. In 1783 he closely followed a large spot, noticing that "it was plainly depressed below the surface of the Sun: and that it had very broad shelving sides ". He was soon led to adopt Wilson's view of the solar constitution, which he developed in his first paper on the Sun in 1794.

His observations of the great spot of 1774 led him to conclude that he viewed "the real solid body of the Sun itself, of which we rarely see more than its shining atmosphere." This interpretation of sun-spot observations was the foundation-stone of his theoretical edifice. He concluded, in agreement with, but independently of, Wilson, that the solar globe was dark and solid, surrounded by a glowing atmosphere composed of various "elastic fluids that are more or less lucid and transparent". This lucid fluid, named by Schröter the "photosphere," Herschel believed to be generated in the Sun's atmosphere. An analogy, he said, might be drawn from the generation of clouds in the terrestrial atmosphere. In his paper of 1801, Herschel concluded that there are "two different regions of solar clouds, like those upon our globe. In that case their light is only the uniform reflection of the surrounding superior self-luminous region."

"The solid body of the Sun beneath these clouds," Herschel said, "appears to be nothing else than a very eminent, large and lucid planet, evidently the first, or in strictness of speaking, the only primary one of our systems; all others being truly secondary to it." This