Circular glass plate photopositive with 44 exposures of what appears to be the planet Venus crossing the Sun's limb. The diameter of the glass plate is 209mm. Two paper labels stuck onto glass give details of the photograph, and the date. The plate is in a buff cardboard box.

Transcribed labels:
Janssen on Model
1874 Nov 13, 1 of [unclear]
Alt .15 from .76 4
Aperture 2.0 inch
1st pass 1.10.21
Last pass 1.11.09
Contact 1.11. 3 or 4 [unclear]

[No] 4 1874 Nov 12
[Collodion] 2
[Preserv] RW
[Preparer] GC [Possibly ROG Assistant George Criswick?]
2

In fact, all 44 images are the same and were made as test plates to check observers knew how to use the new 'revolver' invented by Jules Janssen before they tried it out on the actual Transit of Venus in 1874. In preparation for this rare event, the seventh Astronomer Royal George Biddell Airy ordered 5 English 'Janssens' to be sent to Dallmeyer, who made the specially designed telescopes for photographing the Sun ('photoheliographs').

The triangular nature of the Sun's disc on these 44 images suggest that this plate was used with the transit model that was installed on the roof of Flamsteed House in October 1873. A triangular frame representing both the entry and exit limbs (edges) of the Sun was illuminated by sunlight directed from a rotatable mirror positioned behind it. A metal disc to represent Venus was moved very slowly at less than 1/1000 of an inch per second by a clockwork mechanism. The observers practiced watching the 'event' using telescopes situated around 500 feet (152 metres) away.

A number of test plates like this one exist in collections worldwide, but none of the plates taken of the actual transit with any of Janssen's revolvers are known to have survived.

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.