Oculus in the dome of the Pantheon in Rome. The only source of natural light in the Pantheon, it is 27 feet in diameter and open to the sky (the floor is gently sloped to allow for runoff of rainwater). The first temple on this site was founded in 27 BC by Marcus Agrippa, who is given credit for the building on the portico inscription. But the Pantheon was completely reconstructed in 125 AD by Hadrian, a cosmopolitan emperor who had traveled widely in the East. Hadrian himself is credited with the basic plan, an architectural design that was unique for the time.