Glass photopositive of the Transit of Venus, 1874, as seen from Thebes, Egypt. Circular negative in square wooden frame. 48 exposures of the planet Venus crossing the Sun's limb with number scratched onto the glass every tenth.

Paper label adhered in the centre appears to be a standard photographic label with additional handwritten text that does not match the printed form:

[No.] 6.12 1874
[Collodion] Transit
[Preserv.] RB
[Preparer] AParty

Underneath, the following text has been scratched on:

Thebes
Janssen No. 1
CEB

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.