Circular glass plate negative with 60 exposures of what appears to be Venus crossing the Sun's limb with a number scratched on the glass every tenth. The diameter of the glass plate is 255mm. It consists of a circular negative in glass wooden frame, with a perspex cover on front.

Research at the Royal Observatory, Paris, suggests that this plate is a positive made from the original negative plate taken at Roorkee (India) on the Transit of Venus 1874 between two contacts using a Janssen 'revolver'. However the photoheliographic instrument used at Roorkee - De La Rue's instrument - was only 273mm diameter suggesting the positive might have been enlarged.

The photographic revolver was invented by the French astronomer Jules Janssen (1824-1907) specifically for the Transit of Venus seen in 1874. Attached to a telescope, Janssen's 'revolver' captured a sequence of images to help measure the critical moments of contact as Venus entered and left the Sun's disc. This device was the precursor to the cinema camera.