The mystery of comets' tails has been to some extent penetrated; so far, at least, that, by making certain assumptions strongly recommended by the facts of the case, their forms can be, with very approximate precision, calculated beforehand. We have, then, the assurance that these extraordinary appendages are composed of no ethereal or super-sensual stuff, but of matter such as we know it, and subject to the ordinary laws of motion, though in a state of extreme tenuity. This is unquestionably one of the most remarkable discoveries of our time.

Olbers, as already stated, originated in 1812 the view that the tails of comets are made up of particles subject to a force of electrical repulsion proceeding from the sun. It was developed and enforced by Bessel's discussion of the appearances presented by Halley's comet in 1835. He, moreover, provided a formula for computing the movement of a particle under the influence of a repulsive force of any given intensity, and thus laid firmly the foundation of a mathematical theory of cometary emanations. Professor W. A. Norton of Yale College considerably improved this by inquiries begun in 1844, and resumed on the apparition of Donati's comet; and Dr. C. F. Pape at Altonax gave numerical values for the impulses outward from the sun, which must have actuated the materials respectively of the curved and straight tails adorning the same beautiful and surprising object.