centre of curvature

The distance from P to the centre of curvature is the radius of curvature.

Both arms of the beam carry circular-arched heads, which are similar portions of the complete circles, the centre of curvature being the fulcrum, or axis.

On the side of the beam towards the centre of curvature the longitudinal filaments must be contracted, and on the other side they must be extended.

These reflectors should be spherical and so adjusted that the radiant is in the centre of curvature, thus ensuring that the light from the reflector passes again through the original source.

For instance, if the radius of curvature of the cathode be 1.5 inches, the centre of curvature may lie something like an eighth of an inch or less in front of the anode.