Do the nuclei of comets exhibit phases?–Polarisation of cometary light–Experiments of Arago and of several contemporary astronomers–The light of nebulosities and atmospheres is partly light reflected from the sun.

In the last century astronomers were almost entirely preoccupied with the study of cometary movements, the nature of cometary orbits, the periodicity of comets, and with every question, in fact, that tended to prove that, like the planets, these bodies are subjected to the universal law of gravitation. Astronomical physics was then hardly recognised, and conjecture filled the place of modern analytical research. It was doubtless owing to this preoccupation that comets were at that time looked upon as bodies of kindred nature to the planets. There was a kind of reaction against the ancient hypothesis of terrestrial meteors and transient fires. ‘Planets are opaque bodies,’ says Pingré; ‘they only send back the light which they receive from the sun. We ought not, perhaps, to conclude definitively that comets are also opaque bodies ; it is not absolutely proved that a luminous body may not circulate around some other body. But the light of comets is feeble and dull; its intensity varies; we can perceive in it sensible inequalities and even gaps. It does not appear that these phenomena can be explained otherwise than by supposing comets to be opaque bodies, possessed of no other light than that which they receive from the sun, and surrounded by an atmosphere similar to that of the earth. Clouds are formed within this atmosphere, just as in our own atmosphere; these clouds weaken or totally intercept the rays of the sun, and successively deprive us of the sight of a portion of the comet. This hypothesis would explain everything. . .’