Orders and Classification
Installations were graded by order, a classification based on focal length. A first-order lens, with a focal distance of about 920 millimetres, stood several metres tall, weighed several tonnes, and was reserved for major landfall lights that had to be seen from far offshore. Sixth-order lenses, small enough to carry, marked harbour entrances and river channels.
Assemblies were built as a drum of refracting rings around the focal plane with reflecting prisms above and below to recover light escaping at steep angles. Rotating the whole assembly produced a flashing characteristic; a stationary drum with vertical panels produced a fixed light. Rotation initially ran on chariot wheels driven by clockwork and falling weights, and later floated the assembly on a mercury bath, which nearly eliminated friction at the cost of a serious occupational hazard for the keepers who maintained it.
Because each station could be given a distinct flash interval, the classification system turned individual lights into an identifiable network rather than a set of anonymous glows.