Spent the week out on the north approaches with a borrowed light meter and a notebook that is now mostly seawater. Every keeper I met had a different theory about why the 1953 optic throws a good deal further than the replacement fitted in the eighties, and honestly every one of them turned out to be partly right about it.

Short version: the glass is not the interesting part of the story. The interesting part is how patiently somebody sat on the gallery and aligned it, one prism at a time, until the sweep came out even the whole way round the tower.