Dr. Bernal's original proposal described a hollow spherical shell 16 km in diameter, with a target population of 20,000 to 30,000 people. The Bernal sphere would be filled with air and rotated to provide artificial gravity via centrifugal force, resulting in an interior landscape that would resemble a large valley running all the way around the equator of the sphere. Sunlight was to be provided to the interior of the sphere using external mirrors to direct it in through large windows near the poles. The form of a sphere was chosen for its optimum ability to contain air pressure and its optimum mass-efficiency at providing radiation shielding.
In a series of studies held at Stanford University in 1975 and 1976 with the purpose of speculating on designs for future space colonies, Dr. Gerard Kitchen O'Neill proposed a modified Bernal sphere with a diameter of only 500 m rotating at 1.9 RPM to produce a full Earth gravity at the sphere's equator. This version of the Bernal sphere was dubbed the "Island One" design, with Islands Two and Three being the Stanford torus and O'Neill cylinder, respectively.