According to Wolf, the attraction of the planets, or of some of them, is the real cause of the periodicity which we are dealing with; that attraction producing on the surface of the solar globe true tides, which give birth to the spots, these tides themselves experiencing periodic variations owing to the periodic changes of position of the celestial bodies which cause them. It has even been thought safe to assert that the fact of the principal period coinciding with the revolution of Jupiter is of momentous significance; but this coincidence seems purely accidental, and no certain conclusion can be drawn as to this matter. The influence of Mercury and Venus would perhaps be much more potent, for their distance from the Sun is not very great, and this should render their influence more sensible. On the other hand, their masses appear to be too small to be capable of producing any sufficient effect. De La Rue, Balfour Stewart, and Löwy most perseveringly studied this point of solar physics. They seem to have arrived at the conclusion that the conjunctions of Venus and Jupiter do exercise a certain amount of influence on the number of the spots and on their latitude; and that this influence is less considerable when Venus is situated in the plane of the solar equator. At any rate it is a fact, that a great number of the visible inequalities in a duly plotted curve of the spots do really correspond to special positions of these two planets. In order to determine with more precision these coincidences and the importance which attaches to them, De La Rue extended his inquiries. He separately analysed many different groups of spots, selecting for his purpose more particularly those of which the observations happened to have been specially continuous and complete, giving a preference moreover to those which had been observed in the central portions of the Sun’s disc. From an investigation of 794 groups De La Rue arrived at the following conclusions:—(1) If we take a meridian passing through the middle of the disc and represented by a diameter perpendicular to the equator, we find that the mean size of the spots is not the same with regard to that meridian. It appears certain that the correction required for perspective does not suffice to explain this difference; and that another element must be introduced in order to secure that the apparent dimensions of the spots may be the same on both sides. We do not yet possess a very clear explanation of this fact; but the most probable is this:—the spots are surrounded by a projecting bank, which seems to disappear in part during their transit across the Sun. This bank is more elevated on the preceding than on the following side; accordingly, the spots ought to seem smaller when they are in the eastern half of the disc; larger when they are in the western half; for in the first position the observer’s eye meets an elevated obstacle, which hides a portion of the spot itself.