The earliest of Newton’s great achievements in natural philosophy was his detection of the composite character of light. That a beam of ordinary sunlight is, in fact, a mixture of a very great number of different colored lights, is a doctrine now familiar to every one who has the slightest education in physical science. We must, however, remember that this discovery was really a tremendous advance in knowledge at the time when Newton announced it. [Illustration: SIR ISAAC NEWTON.] A certain story is often told about Newton. It is said that as he sat in a garden an apple fell from a tree, and that its fall set him thinking, and that, in consequence of this train of thought, he found out all about the law of attraction or gravitation. But the fall of an apple could not possibly have set Newton thinking, because he had already been thinking long and hard upon these difficult questions. Indeed, it was _because_ he had been so thinking, _because_ his mind was in a watchful and receptive condition, that so slight a matter as a falling apple should, perhaps, have suggested to him a useful train of ideas. Newton was a man who really did think, and who really did think intensely. He was not, like the ordinary run of people, one who merely had a good many notions walking loosely through his brain. He would bend his mind to a subject and be absolutely absorbed in it; later in life so absorbed that he would forget to finish his dressing, and would be unable to say whether or no he had dined. This sort of forgetfulness does sometimes spring from vacancy of mind; but with Newton it arose from fullness of thought. An apple falls, of course, as a stone or any other body falls, because the earth attracts it. Otherwise, it might as easily rise and float away into the sky. But this was known perfectly well long before Newton’s days. It was clearly understood that the earth had a curious power of drawing all things towards herself. People could not explain why or how it was so; neither can we explain now; but they were fully aware of the fact. So men knew something of the force called gravity; but they knew it mainly, almost exclusively, as having to do with our earth, and with the things upon our earth. They had not begun definitely to think of gravity as having to do with bodies in the heavens; as being a chief controlling force in the whole Solar System; much less as reigning throughout the vast universe of stars. It had not come into their minds with any distinctness that, just as the earth pulls towards her own center a mountain, a horse, a man, so the sun pulls toward his center the earth, the moon, and all the planets.