Not on display
The meaning of this diagram, which incorporates a circle, is unclear. However it seems to represent some kind of instrument, possibly part of a camera obscura. One was installed in John Fuller’s Observatory near Rosehill (now Brightling) Park, which Turner drew in this sketchbook (folios 37 verso–38; D10254–D10255; Turner Bequest CXXXVII 36a–37).
On the back cover (D41535) there is a slight sketch which may also be the beginning of a diagram, although it does not appear to be related to the present sketch.
A small hole half-way down the left-hand edge seems to relate to a former means of fastening the front cover of the sketchbook (see under D40864).