HEMISPHERE

, the half of a sphere or globe, when divided in two by a plane passing through its centre.

Hemisphere is also used for a map or projection of half the terrestrial globe, or of half the celestial spher<*>, on a plane; being more frequently called a planisphere.

The centre of gravity of a Hemisphere, is five-eighths of the radius distant from the vertex.

A glass Hemisphere unites the parallel rays at the distance of four-thirds of a diameter from the pole of the glass.