Bernal's original proposal described a hollow non-rotating spherical shell 10 mi (16 km) in diameter, with a target population of 20,000 to 30,000 people.[1] 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 K. O'Neill proposed Island One, a modified Bernal sphere with a diameter of only 500 m (1,600 ft) rotating at 1.9 RPM to produce a full Earth artificial gravity at the sphere's equator.Island One would be capable of providing living and recreation space for a population of approximately 10,000 people, with a "Crystal Palace" habitat (consisting of several rings attached to the sphere at each pole) used for agriculture.Sunlight was to be provided to the interior of the sphere using external mirrors to direct it in through large windows near the poles.Island Two would be approximately 1800 meters in diameter, yielding an equatorial circumference of nearly six and a half kilometers (four miles).
Interior of a Bernal sphere
Example layout for an Island One-type Bernal sphere