A simple and ancient way to facilitate central stops on projection lenses

This is not something I have done myself, but have discovered in an 1850’s lens I am preparing for sale.
Before the almost universal adoption of Waterhouse stops around 1860, there were two methods of aperture control. Mostly to control depth of field, rather than a wish to lower speed.

The first method was to introduce washer stops in front of of the front lens, either inserted within the lens hood or in an extension of the hood/barrel - usually in a pill box sort of arrangement.

The second method, which was often combined with making it possible to use the front lens independently, was to make use of central stops by having a set of brass washer stops which were screwed or pushed into a mount at the optical center of the lens. This mount was either fixed to one of the two lens cells or had a barrel mounted fixture which could be got at by unscrewing the two halves of the barrel.

This required quite a lot machining work by the original maker and is certain not the path someone wanting to add aperture control to a projection Petzval/ Triplet can follow. But there is no doubt that the provision of stops near the optical centre of the lens is the “correct” optical method.

Another way to provide central aperture control was used in this lens I mentioned. Rather than the complex system developed by lots of Parisian gentlemen in the 1850’s, this was a more simple approach which had the prerequisites of an existing restricting baffle and a screw-in mount of the achromat glass in the front lens.

This simple mechanism consisted of 3 brass prongs soldered to the inside edge of the lens mounting which reach down (almost) to the baffle. These will secure loose washer stops firm against the permanent baffle.

This may seem like a rather low technological solution, unworthy of a well thought of instrument maker (George Knight & Co.) but it works. I think this was an original feature and not a modification.