Plano-convex ingot problematic in the case of the stamped examples as this would require the furnace to be dismantled or else have a short shaft to allow access for stamping (Merkel 1986, Tylecote 1986, 22.) As a solution the furnace could have been tapped into a mould at the completion of smelting. It is possible that both methods were used as several of the ingots seem to have had additional metal poured onto the top in order to allow stamping (Tylecote 1986, 23-4.) Plano-convex ingot Plano-convex ingots are lumps of metal with a flat or slightly concave top and a convex base.
Lens (optics) object off the optical axis of the lens is imaged, where rays pass through the lens at an angle to the axis θ. Rays that pass through the centre of a lens of focal length "f" are focused at a point with distance from the axis. Rays passing through the outer margins of the lens are focused at different points, either further from the axis (positive coma) or closer to the axis (negative coma). In general, a bundle of parallel rays passing through the lens at a fixed distance from the centre of the lens are focused to a ring-shaped