SECTION III - THEORY OF THE IMPULSION OF THE SOLAR BAYS

Summary

Ideas of Kepler concerning the formation of tails–Galileo, Hooke, and Euler–Hypnthesis of Kepler formulated by Laplace–Where does the impulsion come from in the theory of undulations?

Kepler, who for a moment suffered himself to be led away by the idea of Cardan, soon abandoned it, and substituted in its place that of the action of the solar rays. According to this theory cometary tails have substance and are formed of materials borrowed from the comet, its nucleus, or at least its nebulosity. ‘The sun,’ says Kepler, ‘strikes upon the spherical mass of the comet with direct rays, which penetrate its substance, and carrying with them a portion of this matter, issue thence to form that trace of light which we call the tail of the comet. This action of the solar rays rarefies the particles which compose the body of the comet; it drives them away and dissipates them.’

Hooke, a contemporary of Newton, in order to explain the ascent of the light and tenuous matters which, emanating from the nucleus and flowing back in a direction opposite the sun, contribute to form the tail, assumes that these volatile matters are imponderable : to gravitation he opposes their levitation; according to him they have a tendency to fly from the sun. This amounts to assuming a repulsive force without explaining where this force resides. The opinion of Kepler has been completed, extended, and modified.