considers, however, that these forms are not faculæ, but distinct phenomena, to which he proposes to assign the name 'faculides'; and for various reasons his view appears to be the more probable. They are, however, in any case, in close relation with the faculæ, and, as Miss Clerke observes, 'symptoms of the same disturbance.' [Illustration: PLATE VII. Solar Surface with Faculæ. Yerkes Observatory.] The question of the nature of the sun spots is one that at once suggests itself; but it must be confessed that no very satisfactory answer can yet be given to it. None of the many theories put forward have covered all the observed facts, and an adequate solution seems almost as far off as ever. No one can fail to be struck with the resemblance which the spots present to cavities in the solar surface. Instinctively the mind seems to regard the umbra of the spot as being the centre of a great hollow of which the penumbra represents the sloping sides; and for long it was generally held that Wilson's theory, which assumed this appearance to correspond to an actual fact, was correct. Wilson found by observation of certain spots that when the spot was nearest to one limb the penumbra disappeared, either altogether or in part, on the side towards the centre, and that this process was reversed as the spot approached the opposite limb, the portion of the penumbra nearest the centre of the disc being always the narrowest. This is the order of appearances which would naturally follow if the spot in question were a cavity; and if it were invariable there could scarcely be any doubt as to its significance. But while the Wilsonian theory has been recognised in all the text-books for many years, there has always been a suspicion that it was by no means adequately established, and that it was too wide an inference from the number of cases observed; and of late years it has been falling more and more into discredit. Howlett, for example, an observer of great experience, has asserted that the appearances on which the theory is based are not the rule, but the exception, and that therefore it must be given up. Numbers of spots seem to present the appearance of elevations rather than of depressions, and altogether it seems as though no category has yet been attained which will embrace all the varieties of spot-form. On this point further observation is very much needed, and the work that has to be done is well within the reach of even moderate instruments.