Transit of Venus photoheliograph, 1874.
© Science Museum / Science & Society Picture Library
Photograph of a photoheliograph, a photographic telescope, which was used to observe the 1874 Transit of Venus. It was part of the equipment supplied to the French expeditions sent to view and measure this rare event from Japan. It gave astronomers an opportunity to accurately determine the size of the Solar System. This view shows the photoheliograph in its portable observatory building. After a period of over a hundred years the next transits of Venus occur in 2004 and 2012.