Transit of Venus telescope, 1876.
© Science Museum / Science & Society Picture Library
Photograph of an equatorially mounted refracting telescope, of 6 inch aperture, which was used to observe the 1874 Transit of Venus. Made by Troughton & Simms, it was part of the equipment supplied to the five British expeditions, sent around the world to view and measure this rare event. It gave astronomers an opportunity to accurately determine the size of the Solar System. After a period of over a hundred years the next transits of Venus occur in 2004 and 2012.