Janssen's photographic apparatus

In 1874 a telescope (H10211) and this photographic apparatus were brought to Australia especially to photograph the Transit of Venus. They are the same as those sent to five other observatories around the world that were part of the British Royal Observatory Transit of Venus program. The others went to Honolulu, Mokkatam, Rodriguez, Kereguelen and Burnham.

This photographic apparatus was made by J. H. Dallmeyer based on a unique design by Janssen modified by de la Rue in Britain and took 6.5 inch circular photographic plates (H10379). Of all the British sets of apparatus made for the 1874 Transit of Venus this is the only known one to have survived and this adds substantially to its significance.

For the observation of the Transit of Venus the telescope (H10211) and this apparatus were set up at Woodford in the Blue Mountains at the residence of A. Fairfax. There were seven observers present for the occasion: P. F. Adams Surveyor-General; Hirst a well known amateur astronomer; Mr. Vessy of the Trigonomical Survey; Mr. Du Faur of the Survey Department; Mr. Bischoff the photographer and two unnamed carpenters.

Unfortunately of the 14 Janssen plates taken at Woodford none have survived. Twelve of the resulting Jansen photographs (60 on each plate), and 36 normal plates were sent to the Royal Observatory in Greenwich and have since been lost. The whereabouts of the other two is not known although they may have found their way into the New South Wales Government Printing Office. Only one of the unexposed Janssen plates has survived (H10379).

One reason the plates which were sent to England were not well cared for is that, like the other photographs sent in from observatories around the world, the plates proved to be less than successful. The reasons for this were described by George Airy, Astronomer Royal at the Greenwich Observatory, in 1881, "After laborious measures and calculation it was thought best to abstain from publishing the results of the photographic measures as comparable with those deduced from telescopic view. The consideration which led to this decision are هة that, however well the Sun's limb on the photograph appeared to the naked eye to be defined, yet on applying to it a microscope it became indistinct and untraceable"

However while the photographs proved less than successful the observations themselves played an important part in the official report made by Captain Tupman to the British Government. Of the 61 reliable reports of Venus crossing the sun which were recorded at points around the entire British Empire 22 were form Australia.