This is a "diagonal" attachment presumably for a telescope. It is made up of two brass tubes welded at right angles and containing at the junction a plain elliptical mirror (AST0828). The whole construction is mounted on wooden base, which has four iron screws to adjust the level on to iron plate, with a brass clamping bolt through both. The smaller tube is fitted with a thread for a medium sized eyepieces, and an adapter (unthreaded) of smaller size.

It is unclear precisely what this attachement is for, besides reflecting light to a different location. This was not a standard attachement to William Herschel's telescopes, nor would it be for any astronomical telescope since every additional reflection absorbs light making the light from distant stars appear fainter and fainter with each reflection.

It is assumed that this attachement was made for William Herschel to use with one of his telescopes since it was found and catalogued along with other material relating to his telescope making at Observatory House in Slough in 1924.

It is known that Alexander, William's brother, did much of the brass work for William Herschel's telescopes, and certainly made eyepieces and mounts for him. It seems likely that this telescope part was also made by Alexander.