There can be little doubt, however, that the apparent brightness of this lunar crater, or rather of its summit, is due to some peculiar quality in the surface, which may perhaps be covered by some crystalline or vitreous matter poured out in the far distant time when the crater was an active one. Prof. Shaler, who examined the crater when it was illuminated only by earthshine, with the fine 15-inch telescope of the Harvard Observatory (Cambridge U.S.), says that he has been able to recognise nearly all the craters over 15 miles in diameter in the dark part. 'There are several degrees of brightness,' he says, 'observable in the different objects which shine out by the earth-light. This fact probably explains the greater part of the perplexing statements concerning the illumination of certain craters. It certainly accounts for the volcanic activity which has so often been supposed to be manifested by Aristarchus. Under the illumination by the earth-light this is by far the brightest object on the dark part of the moon's face, and is visible much longer and with poorer glasses than any other object there.' Here my record of astronomical myths must be brought to a close. It will be noticed that in every instance either the illusion has affected the actual observations of eminent and skilful astronomers, or has caused such astronomers to put faith for a while in illusory observations. Had I cared to include the mistakes which have been made by or have misled observers of less experience, I could have filled many sheets for each page of the present article. But it has seemed to me more instructive to show how error may affect the observations even of the most careful and deservedly eminent astronomers, how even the most cautious may be for a time misled by the mistakes of inferior observers, especially when the fact supposed to have been observed accords with preconceived opinions. XII. _THE ORIGIN OF THE CONSTELLATION-FIGURES._