POWERHOUSE COLLECTION

Photographic glass plate and metal ring for use with photoheliograph and Janssen's apparatus

Object No. H10379

These five Janssen photographic plates are the only ones which have survived from the New South Wales project to photograph the 1874 Transit of Venus. The circular plates were used in conjunction with Janssen's photoheliograph apparatus (H10213), which was attached to the photoheliograph telescope (H10211). It was hoped that photographs taken of Venus as it moved in front of the face 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 photographs was similar to 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 (Hawaii); Mokkatam (Egypt); Rodriguez Island (east of Mauritius); Kerguelen Island (south Indian Ocean) and Burnham (New Zealand). The photographic apparatus was made by J H Dallmeyer based on a unique design by Janssen and Warren de la Rue and took 11-inch circular photographic plates. For the observation of the Transit of Venus the photoheliograph telescope (H10211) and Janssen's 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; G D Hirst a well-known amateur astronomer; Mr. L A Vessy of the Trigonomical Survey; Mr Du Faur of the Survey Department; Mr. J Bischoff the photographer and two unnamed carpenters. Hirst operated the photoheliograph-Janssen apparatus instrument. Unfortunately, of the 17 Janssen plates exposed at Woodford none have survived. Fourteen of the resulting Janssen plates (with 60 photographs on each plate), and 197 normal plates were sent to the Royal Observatory in Greenwich and have since been lost. The whereabouts of the other three is not known. These five plates may have been among the 30 Henry Russell said were taken to Woodford. However, only one of the five has survived intact, the others having been broken sometime between the early 1980s and 2007. 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. References 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 Russell, H C ''Observations of the Transit of Venus, 9 December 1874'', Government Printer, 1892 Geoff Barker, August, 2007 Updated by Andrew Jacob, Feb 2022