Now, for the first time in more than 70 years, the space beneath the dome — the Barker Library reading room, a 75-foot rotunda with an intricate 27-foot skylight, or oculus — has been restored to its original splendor. “If the Great Dome is MIT’s signature building,” says Stephanie Hartman, a librarian at Barker who has been researching the history of the reading room, “then in some sense, the reading room is MIT’s signature space.”
The restoration of the reading room was just part of a renovation of the dome itself that involved, among other things, lifting off all of its limestone cladding so that a new waterproof membrane could be injected beneath it.
The dome of the reading-room rotunda, whose diameter and height are both 75 feet, is nested inside the 100-foot-wide Great Dome; the two domes converge at the circular oculus, which is set in the center of both. In 1942, the oculus met the wartime fate of many skylights in U.S. cities: It was covered over, to avoid providing an easily identifiable nighttime target for enemy bombers.
During the research phase of the restoration project, which began in 2007, Gary Tondorf-Dick, a restoration architect and manager of special projects for the MIT Department of Facilities, discovered a photo suggesting that in the early 1950s, the oculus had been reopened. But perversely, it was almost entirely obscured by a massive plastic disc suspended about 20 feet above the library floor, the “luminous ceiling” that had been added during a 1953 remodeling. Sometime thereafter, the oculus was sealed again — probably because of leaking, Tondorf-Dick speculates.
At 27 feet in diameter, the oculus is almost exactly the same size as that of the Pantheon in Rome, which was built early in the second century. That’s no accident, Tondorf-Dick says: “Architecturally, in terms of its classicism, [Building 10] was a reinterpretation of a lot of earlier classical forms,” he says.
While the Pantheon’s oculus is simply a hole in the ceiling, the Great Dome’s is glassed in. From the reading-room floor, the oculus appears to be spanned by a single large, round window. But that’s an illusion: In typical MIT fashion, the optics of the oculus are the result of ingenious engineering.
The glazing in the Great Dome actually consists of 1,042 blocks of glass, each 6 1/4 inches to a side and 1 1/4 inches thick. The blocks are grouped into six-by-six squares; within each square, the blocks are spaced 2 1/2 inches from each other. The borders between the six-by-six squares are thicker still, so that in fact, much of the 27-foot-wide opening in the dome is occluded by structural supports, while the pattern formed by the blocks is at best a boxy approximation of a circle, like something out of an early video game.
The glass blocks, however, are embossed with patterns that concentrate the intensity of the light passing through them by about 15 percent, in the manner of a Fresnel lens. A couple of feet below the blocks, spanning the breadth of the oculus, is a second layer of glazing, a “lay light” with structural supports arranged as radii and concentric circles. The panes of the lay light are a prismatic glass that diffuses the light from above them so that, seen from below, all but the largest structural elements of the dome’s glazing seem to disappear.
Originally, the glass blocks were embedded in a concrete matrix; during the renovation, that was replaced with a stainless-steel frame so that in the future, damaged blocks can be swapped out individually. According to Tondorf-Dick, reproducing the light-focusing glass blocks required coordinating the work of three different vendors: one that created new molds from a few surviving specimens; one that developed the chemistry for the glass, so that it could be manufactured in inch-thick slabs and would survive exposure to the elements; and one to handle the bulk fabrication.
The reopening of the oculus means that the glorious architectural details of the rotunda can once more be seen distinctly; among these are the recessed coffers of the ceiling, very much like those of the Pantheon; the Corinthian columns ringing the room; and the acanthus leaves around the base of the dome.
When Building 10 opened in 1916, the architectural detail of the rotunda was further emphasized by a subtle color scheme that involved seven different shades of off-white and a green that simulates oxidized bronze. “But when we started, everything had been painted out MIT white,” Tondorf-Dick says. After much research — including paint-sample analysis and computer simulation — the color scheme has been restored, as has the plasterwork of the walls and dome. High-efficiency LED lights have also been added around the oculus and between the columns, and upturned floodlights that illuminate the dome are concealed behind the acanthus leaves.
“It’s night and day,” Hartman says. “It was very dark before: The dome just kind of receded into the background. It got a nickname at some point: the Barker batcave.” But since the renovation, she says, “It’s much brighter. The space is much livelier than it was before.”
Earlier this week, the last of the scaffolding came down from the exterior of the Great Dome. Starting Feb. 27, the reading room will be open 24 hours a day, seven days a week to members of the MIT community.