Rapid undulations occasionally observed in the light of cometary tails ; observations of Kepler, Hevelius, Oysatus, and Pingre; comets of 1607, 1618, 1652, 1661, and 1769–Undulations in the tails of the comets of 1843 and 1860; do these undulations arise from a cause peculiar to the comet itself, or do they depend upon the state of the atmosphere?–Objection made by Olbers to the first of these hypotheses ; refutation by M. Liais.

The tails of certain comets have exhibited variations of brilliancy, sudden changes of intensity, analogous to the phenomena of the same kind which are observed in the aurora borealis, and which, it is believed, have been remarked in the zodiacal light. This fact was unknown to the ancients ; and when Seneca speaks of the augmented or diminished brilliancy of comets, it is evident that he alludes to the changes produced, in the course of their apparition, by the variations of their distance from the earth. He compares them ‘to other stars which throw out more light and appear larger and more luminous in proportion as they descend and come nearer to us, and are smaller and less luminous as they are returning and increasing their distance from us.’ (Quæstiones Naturales, vii. 17.)

Kepler is the first observer who has made mention of these singular changes.