At a later date, as we shall presently see, this peculiarity would have been seized upon as the suggestion of another type of dome, of which I shall have subsequently to treat. As a matter of fact, the transition from the polygonal prism below to the nearly hemispherical dome above, is got over by “rule of thumb,” rather than on any true system. The vertical sides of the wall do intersect the dome in arched forms; but neither are these forms the true sections of a plane with a sphere, nor have they been used as architectural features, as in later times; but have been afterwards, so far as I can judge, obliterated by the incrustation of the dome with plaster, so as to slur over a union of forms which the architect had fallen into accidentally without appreciating its true results. [Illustration: Fig. 402.--Minerva Medica. Plan and Section.] The dome is surrounded by gradenæ much as in the Pantheon. It is not lighted by an eye, but by ten windows, surrounding what we should call the clerestory. Beneath these are ten arches piercing the surrounding wall; indeed, reducing it to small anglepiers. One of these was devoted to the entrance, the other nine to semicircular recesses, of which five seem to have contained basins for water, and four to have opened by means of colonnades into exedræ or surrounding buildings.[63] It may be mentioned that this form was in after times extensively imitated. The span of this dome is about 80 feet. Its date is not known. [Illustration: Fig. 403.--Diocletian’s Palace at Spalatro. (_From Fergusson._) A. Temple of Jupiter.] There are other domes not differing materially from those already described, but which it would extend my lecture unduly to dwell upon. One called the Torre dei Schiavi, in the Via Prænestina, is rather like the Pantheon on a very small scale, though lighted by round clerestory windows instead of a central eye. The Temples of Vesta, both at Rome and at Tivoli, consist of circular walls surrounded externally by a peristyle. The cell of each is supposed to have been covered by a dome, though roofed over. A parallel case, but in a more complex form, exists in what is called the Temple of Jupiter in Diocletian’s Palace at Spalatro (Fig. 403). In this case the exterior of the cell, with its peristyle, is octagonal, but the interior, with the dome, _round_. The latter has a complicated construction of fan-shaped arches throughout, scarcely any part being constructed of horizontal curves. Next, perhaps, in date, yet at once displaying similarity of idea with a significant change in the carrying out, is the tomb of St. Constantia, the daughter of Constantine (Fig. 404). [Illustration: Fig. 404.--Tomb of St. Constantia, Rome. (_From Fergusson._)]