Janssen photographic plate and metal ring in storage box, for use with photoheliograph and Janssen's apparatus, glass / metal / wood, maker unknown, used by Sydney Observatory, Sydney, New South Wales, Australia, 1874
This unexposed Janssen photographic plate is the only one which has survived from the New South Wales project to photograph the 1874 Transit of Venus. These circular plates were used in conjunction with Janssen's photoheliograph apparatus (H10213), which was attached to the transit telescope (H10211). It was hoped that photographs taken of Venus as it travelled over the surface of the sun would lead to a more precise measurement of the distance of the Earth from the Sun.
The Janssen apparatus that was used to take took these circular photographs was the same as those sent to five other observatories around the world. These were part of the British Royal Observatory Transit of Venus program and were situated at; Honolulu; Mokkatam; Rodriguez; Kereguelen and Burnham. The photographic apparatus was made by J. H. Dallmeyer based on a unique design by Janssen and de la Rue and took 6.5 inch circular photographic plates.
For the observation of the Transit of Venus the telescope (H10211) and apparatus (H10213) 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 this unexposed Janssen plate has survived the others having been broken sometime between the early 1980s and 2007.
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 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 from Australia.
Todd, David, P., Stars and Telescopes, Sampson Low, Marston, and Co., 1900
De-Clerq, P.R., Nineteenth Century Instruments and their Makers; Rodopi, Amsterdam, 1985
Airy, G. B, Account of the Observation of the Transit of Venus, 1874, December 8, Made Under the Authority of the British Government and of the reduction of the Observations, Her Majesty's Stationary Office, 1881
Russell, H., C., "Report of Astronomer for 1874 & 1875', New South Wales Government Printer, 1876
Knight, E., H., (ed), 'Knights American Mechanical Dictionary', Vol III, J.B. Ford and Company, New York, 1874
Geoff Barker, August, 2007