from an effect of perspective which was easy to be understood. The different phases presented by such a spot as that just spoken of will be so much the more sensible according as the depth of the cavity is greater; but if the depth is inconsiderable the bottom of the cavity will only disappear when a very oblique angle is attained, and this cannot happen except when the spot is very near to the limb. By observations carefully made under such circumstances it will be possible to determine the depth of the cavity, and Wilson found that the depth of a spot often amounted to about one-third of the Earth’s radius. Wilson’s theory was not accepted without dispute; it was contested by several astronomers, and in particular by Lalande. It was however taken up by Sir W. Herschel, and as modified by him has met with general acceptance down to the present time; though now and again challenged, perhaps most recently and most vehemently by Howlett, a sun spot observer of great experience. Wilson’s discovery was the point of departure for the grand labours of Sir W. Herschel in the field of Solar Physics. Man of genius that Herschel was, he was above all things an observer who took his own line in what he did. He saw so many phenomena with the powerful instruments constructed by himself, he described so minutely the marvels which were revealed to him, that he left comparatively little for his successors to do so far as regards mere telescopic observation. Herschel’s main idea as to the Sun was based on Wilson’s discovery. He remarked with reason, as that astronomer had done, that if the spots are cavities the luminous matter could neither be properly called liquid nor gaseous; for then it would precipitate itself with frightful rapidity to fill up the void, and that would render it impossible that the spots should endure as we often see they do during several revolutions of the Sun. Moreover, the proper movements of the spots prove that the photosphere is not solid. We can therefore only liken it to fogs or clouds, and it must be suspended in an atmosphere similar to ours. Such is, according to Herschel, the only hypothesis which can explain the rapid changes which we witness. We shall see a little later on that these phenomena do admit of another explanation.