Waterhouse stop

A thin piece of metal (the diaphragm) is drilled with a hole (the aperture); a set of these with varying hole sizes makes up a set of Waterhouse stops, corresponding to what today we call f-stops or f-numbers.

This apparatus was invented by the pioneering 19th-century photographer John Waterhouse of Halifax in 1858.

[3][4] The innovation was quickly put to use due to its convenience: "Aperture openings were at first controlled by unscrewing the lens and inserting stops of the appropriate size between the lens components, though after 1858 photographers used the more convenient Waterhouse stops which eliminated unscrewing the components.

Such multi-aperture diaphragms were also sometimes referred to as Waterhouse stops, due to their operation based on sliding through a slot in the lens barrel.

Today, Waterhouse stops are largely obsolete; most modern photographic lenses are made with an iris diaphragm.

Two views of an early Dallmeyer Soft Focus Series B with a focal length of 8½" and fast maximum aperture of f / 3, with a Waterhouse stop shown inserted (left) and removed (right).
Cassell's Cyclopedia of Photography 1911 identifies John Waterhouse of Halifax as the inventor in 1858 of Waterhouse stops, "a separate stop being required for each opening."