The variations of the spots which we now recognise naturally recall those obscurations of the Sun which are recorded in history; but it is necessary to accept many of these with caution. A great number of these phenomena which attracted the attention of people in early times are only eclipses badly observed and still more badly described. In other instances the obscuration has been produced by very protracted dry fogs. It is probably to this last-named cause that we must ascribe the obscuration which, according to Kepler and Gemma Frisius, took place in 1547. It was in some such way as this that, according to Virgil (Georg. i, 630), who has echoed a tradition which he found in history, the Sun was obscured at the death of Cæsar:— Ille etiam extincto miseratus Cæsare Romam Quum caput obscura nitidum ferrugine texit, Impiaque æternam timuerunt sæcula noctem. In the year 553 A.D., and again in the year 626 A.D. the Sun remained obscured for several months; but these facts (if facts they are) besides being ill-observed, and clothed, no doubt, in extremely exaggerated language, are brought to our notice as having occurred at epochs which are quite independent of one another, whilst the variations in the markings on the Sun, which we have just been talking about, present an almost mathematical regularity of sequence. We must now institute some inquiries as to the causes of the periodicity of the spots. A periodicity so well established would naturally invite astronomers to seek the causes which produced it. The presence of spots only in the Zodiacal regions led Galileo to suspect the existence of some relation between the spots and the position of the planets; but there is in this a mere surmise, which, when it was made, had nothing to justify it, and it is still impossible for us to say anything for certain on the point. The determining cause of the periodicity may exist in the interior of the Sun, and may depend on circumstances which will for ever remain unknown to us. Or it may be something external: it may be due after all to the influence of the planets. It remains for us, therefore, to search and see if any such influence can be traced.