all the concrete had been poured and strengthened with internal post-tensioning cables, however, the curved parts of the shells carried the weight of their lower straight edges instead of the other way around. To prevent the shells from collapsing at the long light slots at their apexes, concrete struts were inserted at 10-foot (3 m) intervals. A relatively thick concrete arch was added to each end of the shells to stiffen them further. To make it clear that the curved shells are supported only at their four corners and not by the walls at the ends of the vaults, thin arcs of transparent material were inserted between the curve of the shells and the end walls. Because the stiffening arches of the shells are thicker at the top, the transparent strips are tapered, thinner at the top than at the bottom. In addition, a linear transparent strip was placed between the straight bottoms of the shells and the long exterior walls to show that the shells aren't supported by those walls either. In addition to revealing the building's structure, these features bring additional natural light into the galleries in a way that is safe for the paintings. The vault roofs, which are visible to approaching visitors, were covered with lead sheathing inspired by the lead covering of the complexly curved roofs of the Doge's Palace and St. Mark's Basilica in Venice, Italy. Skylights David Brownlee and David DeLong, authors of Louis I. Kahn: In The Realm of Architecture, declare that "in Fort Worth, Kahn created a skylight system without peer in the history of architecture." Robert McCarter, author of Louis I. Kahn, says the entry gallery is "one of the most beautiful spaces ever built", with its "astonishing, ethereal, silver-colored light." Carter Wiseman, author of Louis I. Kahn: Beyond Time and Style, said that "the light in the Kimbell gallery assumed an almost ethereal quality, and has been the distinguishing factor in its fame ever since." Creating a natural lighting system that has evoked such acclaim was challenging, and Kahn's office and the lighting designer Richard Kelly investigated over 100 approaches in their search for the proper skylight system. The goal was to illuminate the galleries with indirect natural light while excluding all direct sunlight, which would damage the artwork. Richard Kelly, lighting consultant, determined that a reflecting screen made of perforated anodized aluminum with a specific curve could be