Marconi MkIII Television Camera Diascope type BD847
The diascope for the Marconi MkIII was made by "W Watson & Sons"
It has a slide carrier with two 35mm slide positions. the lamphouse has a 12V 6W S.B.C. cap bulb supplied from the camera utility mains outlet via a small transformer.
The slide in the picture with concentric circles is for setting the tube scan sizes together with a check on scan linearity.
The left picture shows the marconi lens mount detail with the hole for the alignment spigot.