|image by Gordon Coale (Image rights)|
Waterhouse stops are a set of holes pierced through a plate within a lens, used to select the aperture. This is a simpler and cheaper method of obtaining a variable aperture than the iris diaphragms used on more advanced cameras.
Waterhouse stops were common in the "brass barrel" era of camera lenses, and are named after British astronomer John Waterhouse. In the late 1850s, Waterhouse invented a system using plates with different aperture diameters that could be inserted into a slot in the lens barrel. This was significantly more convenient than unscrewing the lens barrel to insert discs with different apertures. A rotating wheel with several aperture openings ('aperture dial') was a later refinement.
|Dallmeyer lens with cased set of stops|
image by Geoff Harrisson (Image rights)