must hold good, does hold good for the circular orbit; sees also from Kepler's third law that if it can be applied to the elliptical orbit as well then all the planets immediately fall into that celestial harmony predicted by Pythagoras two millenniums before. The law holds for the circle, does it not hold for the ellipse also? We can imagine with what nervous eagerness Halley argues the question with Wren and Hooke and then—Heaven-inspired thought—posts him down to Cambridge to consult Newton. This was in August, 1684.
We must now rapidly review Newton's own work during the twenty years following his graduation from Cambridge in 1665. From the