_Crateriform Structure._—In 1769 Prof. Wilson, of Glasgow, while watching a sun-spot with a Gregorian reflecting-telescope, remarked that, as it approached near the limb, the penumbra became much foreshortened on the interior side. He inferred from this that the spots were cavities, and the idea has been generally accepted; so that these objects are sometimes termed solar craters, and commonly regarded as openings in the luminous atmosphere of the Sun. But the conclusion appears to be based on data not uniformly supporting it. In 1886 the Rev. F. Howlett published some observations which “entirely militate against the commonly received opinion that the spots are to any extent sunk in the solar surface as to produce always those effects of perspective foreshortening of the inner side of the penumbra (when near the limb) which have been described in various works on astronomy.” In a number of instances the penumbra is wider on the side nearest the Sun’s centre, whereas the converse ought to be the case on the cavity theory. The fine sun-spot of July 1889 offered an example of this; for when it was near the W. limb the W. side of the penumbra was obviously much narrower than the E. side, so that the appearance would indicate the object as an elevation rather than a depression. The observer should keep a register of the aspect of all pretty large spots near the limb, and note the relative widths of the E. and W. sides of the penumbra. An extensive table of such results would be interesting, and certain to throw some light on the theory of spot-structure. It is of course possible that occasionally the inner side of the penumbra is broader than the outer, and thus appears wider even on the limb, though really forming the side of a shallow depression.