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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.

The source and screen are partly independent, and each is provided with a fine adjustment which serves to place it in position near the centre of curvature.