Without this Law of Gravitation the Earth could not retain her spherical shape: only through this constant drawing in toward the center could she exist.
The other planets, too, must be round or they could not exist, and so they also had this same quality of gravity in common with the Earth—a drawing in of everything toward the center. Here was clearly a positive discovery—this similarity of the heavenly bodies!
Every one of the heavenly bodies was exerting a constant attraction toward all other heavenly bodies, and this attractive power must be in proportion to the distance they were from the object acted upon. Thus were their movements and orbits accounted for.
At this time Newton was perfectly familiar with Kepler's Law, that the squares of the periodic times of a planet were as the cubes of its distance from the sun. And from this, he inferred that the attraction varied as the square of the planet's distance from the sun.
Here he was working on territory that had never been surveyed. At first, in his exuberance, he thought to figure out the size and weight of each planet quickly by measuring its attractive power. He did not realize that he had cut out for himself work that would require many men and several centuries to cover, but surely he was on the right scent—a finite man keen upon the secrets of the Infinite!
He was still at his mother's old home in the country, without scientific apparatus or the stimulus of colleagues, when we find by a record in his journal that antique groan because there were only twenty-four hours in a day, and that eight were required for sleep and eight more for recreation!
A subject a little nearer home than planetary attraction had now switched him off from measuring and weighing the stars. He was hard at work in his mother's little sitting-room, with the windows darkened, much to that good woman's perplexity.
By shutting out all light from the windows and allowing the sun's rays to enter by a little, circular aperture, he had gotten the sunlight captured and tamed where he could study it. This ray of light he examined with a small hand-glass he himself had made. In looking at the ray, quite accidentally, he found it could be deflected and sent off at will in various directions. When thrown on the wall, instead of being simply white light it had seven distinct colors beginning with violet and running down to red. So white light was not a single element: it was made up of various rays which had to be united in order to give us sunlight.
Eureka! He had found the secret of the rainbow—the sun's rays broken up and separated by the refracting agency of clouds!
Well does Darwin declare that the separation of sunlight into its component parts, and the invention of the spectrum, have marked an advance in man's achievement such as the world had not seen since the time of wonder-working Archimedes.