That theories of the physical appearances of comets have generally failed, appeared to the author to arise from the facts having been misunderstood or misinterpreted in general by the observers themselves. As a particular instance of this, the wide-spread notion of comets shooting forth their tails, at, or a little before the perihelion passage, and drawing them in again afterwards, so as to be larger at that period of their orbits than at any other, was mentioned; and in place of which, the author shewed that the comets were at the perihelion, of their smallest size; the tails becoming then more visible, not from being actually produced at that time, but from being more dense, and illumined by a stronger solar light, as well as being in general seen from a smaller terrestrial distance.