He studied carefully the nature of the sun, its probable gaseous surface, and its spots, and was the first to suspect their periodic character. What would Herschel have said to the wonderful photographic representations of these spots given by Professor Langley, in his New Astronomy; spots which are one billion square miles in size; more than five times the surface of the land and water on the earth? He saw, as astronomers to-day see, that heat cannot be produced without expenditure of force; and that the sun is probably cooling, even though scarcely perceptibly for ages to come. He saw what science now generally concedes, the rise and fall of the solar system; its gradual fitness for the coming of man, through almost countless centuries; and its final unfitness, when his generations shall have gone forever.
He wrote much about the Milky Way, believing at first that it could be completely resolved into stars, about eighteen millions of them; but later he changed his theory, having found so much nebulous matter—in a state of condensation as though new worlds were forming, possibly to be the homes of some new race, or of man in the ages to come.
His study of the variable stars attracted wide attention. He found that the star Mira Ceti was for several months invisible to the naked eye; then it grew brighter and brighter, and finally disappeared for months, as before. He saw that other stars are periodic, and came to the conclusion that this is occasioned by the rotation of the star upon its axis, by which different parts of its surface are presented to us periodically.
He made a catalogue of double stars, and found by laborious calculations that such stars have a common centre of gravity; that one sun revolves about another. He found that our solar system has a motion of its own; a grand orbit round some as yet unknown centre, and that other systems have a like motion.
What this centre may be, whether a great sun like Sirius, one hundred times larger than ours, with unknown powers and unknown uses, is of course only conjecture.
Herschel gave much attention to nebulæ, discovering and describing twenty-five hundred new nebulæ and clusters. He gave his life to the study of the construction of the heavens. Concerning his statement of the general construction, Professor Holden, himself a brilliant astronomer, says: "It is the groundwork upon which we have still to build.... As a scientific conception it is perhaps the grandest that has ever entered into the human mind. As a study of the height to which the efforts of one man may go, it is almost without a parallel.... As a practical astronomer he remains without an equal. In profound philosophy he has few superiors. By a kindly chance he can be claimed as the citizen of no one country. In very truth his is one of the few names which belong to the whole world."
The distinguished man, though unassuming and gentle in manner, must have had a realizing sense of the greatness of his work, for he said, "I have looked further into space than ever human being did before me. I have observed stars of which the light takes two millions of years to travel to this globe."
He gave much study to light and heat. So boundless was his knowledge believed to be, that a farmer called one day to ask the proper time for cutting his grass.
"Look at that field," said the scientist, "and when I tell you it is mine, I think you will not need another proof to convince you that I am no more weatherwise than yourself or the rest of my neighbors."
He worked earnestly till he was seventy-six, always depending upon his faithful and inseparable Caroline for aid in his labors. He made a telescope for her, with which she swept the heavens for comets, finding eight, five of which she discovered for the first time.