Thomas Black was an astronomer attached to the Greenwich Observatory, so brilliantly presided over by Professor Airy. Mr Black was no theorist, but a sagacious and intelligent observer; and in the twenty years during which he had devoted himself to astronomy, he had rendered great services to the science of ouranography. In private life he was a simple nonentity; he existed only for astronomy; he lived in the heavens, not upon the earth; and was a true descendant of the witty La Fontaine’s savant who fell into a well. He could talk of nothing but stars and constellations. He ought to have lived in a telescope. As an observer be had not his rival; his patience was inexhaustible; he could watch for months for a cosmical phenomenon. He had a specialty of his own, too; he had studied luminous meteors and shooting stars, and his discoveries in this branch of astronomical science were considerable. When ever minute observations or exact measurements and definitions were required, Thomas Black was chosen for the service; for his clearness of sight was something remarkable. The power of observation is not given to everyone, and it will not therefore be surprising that the Greenwich astronomer should have been chosen for the mission we are about to describe, which involved results so interesting for selenographic science. We know that during a total eclipse of the sun the moon is surrounded by a luminous corona. But what is the origin of this corona? Is it a real substance? or is it only an effect of the diffraction of the sun’s rays near the moon? This is a question which science has hitherto been unable to answer. As early as 1706 this luminous halo was scientifically described. The corona was minutely examined during the total eclipse of 1715 by Lonville and Halley, by Maraldi in 1724, by Antonio de’Ulloa in 1778, and by Bonditch and Ferrer in 1806; but their theories were so contradictory that no definite conclusion could be arrived at. During the total eclipse of 1842, learned men of all nations—Airy, Arago, Keytal, Langier, Mauvais, Otto, Struve, Petit, Baily, &c.—endeavoured to solve the mystery of the origin of the phenomenon; but in spite of all their efforts, “the disagreement,” says Arago, “of the observations taken in different places by skilful astronomers of one and the same eclipse, have involved the question in fresh obscurity, so that it is now impossible to come to any certain conclusion as to the cause of the phenomenon.” Since this was written, other total eclipses have been studied with no better results.