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��THE SCIENTIFIC MOyTHLY
��bola are not closed curves, and the branch upon which the comet ap- proaches the sun and the branch upon which the comet recedes from the sun never come together, no matter how far out from the sun they be drawn.
There have been two hypotheses as to where the comets come from. Sir Isaac Xewton thought of them as moving in elongated ellipses. It
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��Fig. 1. Characteristic Forms of Orbits.
��Nvas the view of Immanuel Kant 160 years ago that comets are bona fide members of the solar system, just as the earth and Neptune are : that their orbits are all ellipses, but very elongated ellipses. He said that the comets travel out a great distance from the sun, but that they must Eventually return because they are moving in ellipses. Eiuit^s view of the subject was essentially a mere opinion, though the opinion of one of the greatest philosophers of all time, who gave careful consideration to every known fact. Up to Kant's day, and for many decades later, comet observations were crude in comparison with present-day stand- ards. Most comets were observed for only a few weeks, and the true characters of their orbits could not be affirmed.
Half a century later the great Laplace championed the view that the comets belong to the stellar system and not to the solar system; that comets are travellers through interstellar space; that the wander- ings of a chance few comets bring them within the sphere of influence of our sun; and that we see those which come into favorable position near the earth. Halley's celebrated comet was the only one then known to return again and again to the region of the sun, and it was thought to be a captured wanderer. In Laplace's time also the comets were still inaccurately observed, over short periods of time, and in nearly
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