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peared at the limb of the sun. This happened fifteen minutes before the sun disappeared from view. During this time, if the nucleus were a solid body, it ought to have been seen as a black spot projected against the sun. Nothing of the sort could be made out. The conclusion is either that the substance of the comet was transparent to the sun's rays, or that the solid nucleus was too small to be distinguished under the circumstances. Unfortunately, owing to the low altitude of the sun and the bad condition of the air, it was impossible to be quite sure how small the nucleus must have been to be invisible. It seemed certain, however, that the solid portion, if any such the comet had, was much smaller than the apparent nucleus as seen in the telescope.

There seems also to be some reason for suspecting that a comet is nothing but a collection of meteoric matter, consisting perhaps of separate objects, of sizes ranging anywhere from that of grains of sand to masses as large as the meteorites which sometimes fall from the sky. The question then is to explain how the parts are kept together through so many revolutions of the comet. The changes of shape which the nucleus often undergoes as it is passing near to the sun seem to show that this hypothesis may be near the truth.

The spectra of those comets whose light has been analysed by the spectroscope are remarkable in showing that this light is not merely reflected sunlight. The principal feature is three bright bands, which bear a striking resemblance to those given by the compounds of carbon and hydrogen. Taking this fact by itself, the