Note 26 — Chapter V

There are other laws, besides that of the inverse square of the distance, which would cause a body to move in an ellipse, at least if the force acting on it were placed, not in the focus, but in the centre of the orbit. The question has been discussed with reference to some of the binary stars which appear to move round one another in ellipses. No doubt is thereby raised as to the prevalence of the law of the inverse square in our own solar system, where it has been verified by long and careful observation; the doubt (I think we may say a comparatively slight one) is whether the same law extends to the whole stellar universe, where, of course, accurate observation is impracticable.