The floor is divided into four rooms by party-walls which are probably older than 1475, but which are proved, by the catalogue of 1481, to have been in existence at that period. The first of these rooms, entered directly from the court, contained the Latin Library; the second, the Greek Library. These two, taken together, formed the Common, or Public, Library (Bibliotheca communis, B. publica, or merely Bibliotheca). Next to this room, or these rooms, was the Bibliotheca secreta or Reserved Library, in which the more precious MSS. were kept apart from the others. The fourth room, which was not fitted up till 1480 or 1481, was called Bibliotheca pontificia. In addition to MSS. it contained the papal archives and registers (Regesta). In the catalogue dated 1512 it is called Intima et ultima secretior bibliotheca, and seems to have contained the most valued treasures. This quadripartite division is commemorated by Aurelio Brandolini (Epigram xii.)[372]. After alluding to the founders of some of the famous libraries of antiquity, he says in conclusion:

Bibliotheca fuit, fateor, sua cuique, sed vna.
Sixte pater vincis: quatuor vnus habes.

Thanks to the care with which Platina set down his expenditure, we are able to follow step by step the gradual transformation of the rooms. His account-books[373], begun 30 June 1475, record, with a minuteness as rare as it is valuable, his transactions with the different artists and workmen whom he thought proper to employ. It was evidently intended that the library should be beautiful as well as useful, and some of the most celebrated artists of the day were set to work upon it.

The librarian prudently began in August, 1475, by increasing the light, and a new window was made "on the side next the court." It seems to have been impossible to get either workmen or materials in Rome; both were supplied from a distance. For the windows, glass, lead and solder were brought from Venice, and a German, called simply Hormannus, i.e. Hermann, was hired to glaze them. For the internal decoration two well-known Florentine artists—the brothers Ghirlandajo—were engaged, with Melozzo da Forli, who was painting there in 1477[374]. In 1476 the principal entrance was decorated with special care. Marble was bought for the doorcase, and the door itself was studded with 95 bronze nails, which were gilt, as were also the ring and knocker, and the frame of trellised ironwork (cancellus), which hung within the outer door.

The building is entered from the Cortile del Papagallo[375] through a marble doorway ([fig. 98], A) in the classical style surmounted by the arms of Sixtus IV. On the frieze are the words sixtus papa iiii. The doorcase is doubtless that made in 1476; but the door, with its gilt nails and other adornments, has disappeared. Within the doorway there has been a descent of three steps at least to the floor of the Library[376]. The four rooms of which it was once composed are now used as the Floreria or Garde-meuble of the Vatican Palace; a use to which they have probably been put ever since the new Library was built at the end of the sixteenth century.

The Latin Library, into which the door from the court opens directly, is a noble room, 58 ft. 9 in. long, 34 ft. 8 in. wide, and about 16 ft. high to the spring of the vault. In the centre is a square pier, which carries the four plain quadripartite vaults, probably of brick, covered with plaster. The room is at present lighted by two windows (B, C) in the north wall, and by another, of smaller size, above the door of entrance (A). That this latter window was inserted by Sixtus IV., is proved by the presence of his arms above it on a stone shield. This is probably the window "next the court" made in 1475. The windows in the north wall are about 8 ft. high by 5 ft. broad, and their sills are 7 ft. above the floor of the room. Further, there were two windows in the west wall (b, c) a little smaller than those in the north wall, and placed at a much lower level, only a few feet above the floor. These were blocked when the Torre Borgia was built by Alexander VI. (1492-1503), but their position can still be easily made out. This room must have been admirably lighted in former days.

The room next to this, the Greek Library, is 28 ft. broad by 34 ft. 6 in. long. It is lighted by a window ([fig. 98], D) in the north wall, of the same size as those of the Latin Library, and by another (ibid., E) a good deal smaller, opposite to it. This room was originally entered from the Latin Library by a door close to the north wall (d). But, in 1480[377], two large openings (e, f) were made in the partition-wall, either because the light was found to be deficient, or because it was thought best to throw the two rooms into one as far as possible. At some subsequent date the door (d) was blocked up, and the opening next to it (e) was carried down to the ground, so as to do duty as a door. The other opening (f), about 7 ft. 6 in. square, remains as constructed.

The decorative work of the brothers Ghirlandajo can still be made out, at least in part, though time has made sad havoc with it. The edges of the vaulting were made prominent by classical moldings coarsely drawn in a dark colour; and at the key of each vault is a large architectural ornament, or coat of arms, surrounded by a wreath of oak-leaves and acorns, to commemorate the Della Rovere family. They are tied together on each side with long flaunting ribbons, which, with their shadows, extend for a considerable distance over the vaults. The semi-circular lunettes in the upper part of the wall under the vaults are all treated alike, except that those on the sides of the room, being larger than those at the ends ([fig. 98]), contain two subjects instead of one. The lower part, for about 3 feet in height, is painted to represent a solid marble balcony, behind which a Doctor or Prophet is supposed to be standing. He is visible from rather below the waist upwards, and holds in his hand a scroll bearing an appropriate text. On each side of the figure in the smaller lunettes, resting on the balcony, is a large vase of flowers; and behind it a clear sky. Round the upper edge of the lunette is a broad band of oak-leaves, and fruits of various kinds. The figures, of which there were evidently twelve originally, are the following, beginning with the one at the north-east corner over the door leading into the Greek Library, and proceeding to the right:

1.Hieronymus.Scientiam scripturarum ama, et vitia carnis non amabis.
2.Gregorius.Dei sapientiam sardonyco et zaphyro non confer.
3.Thomas.Legend illegible.
4.Bonaventura.Fructus scripturæ est plenitudo æternæ felicitatis.
5.Aristoteles.}Legends illegible.
6.Diogenes.
7.Cleobulus.
8.Antisthenes.
9.Socrates.
10.Plato.
11.Augustinus.Nihil beatius est quam semper aliquid legere aut scribere.
12.Ambrosius.Diligentiam circa scripturas sanctorum posui.

Jerome and Gregory occupy the east wall; Thomas Aquinas and Bonaventura the first lunette on the south wall, over the door of entrance; Aristotle and Diogenes the next, succeeded by Cleobulus and Antisthenes on the west wall; on the first lunette on the north wall are Socrates and Plato; in the second Augustine and Ambrose, facing Aquinas and Bonaventura. Thus the eastern half of the library was presided over by doctors of the Christian Church, the western by pagan philosophers.