Stereoscopic photograph of inflated bullock skin boat, or dreas, at the side of the river Sutlej, in Himachal Pradesh, taken by James Ricalton in c. 1903, from The Underwood Travel Library: Stereoscopic Views of India. This image is described by Ricalton in 'India Through the Stereoscope' (1907): "I have crossed the river several times on these inflated bullock-skins...The drea-man, after inflating the skin as you see them doing here, places it on the water and places himself on his stomach athwart the skin with his feet in the water; he holds a short paddle in his hands. The intending passenger sits erect, astride the drea-man...You have observed how the skin for this purpose is taken from the animal in one piece and how all openings in the skin are closed except in one leg which is kept open for inflation...These drea-wallahs can drive the skins across the river during high floods when the best swimmer would be helpless in the powerful current." One of a series of 100 photographs that were supposed to be viewed through a special binocular viewer, producing a 3D effect. The series was sold together with a book of descriptions and a map with precise locations to enable the 'traveller' to imagine that he was really 'touring' around India. Stereoscopic cameras, those with two lenses and the ability to take two photographs at the same time, were introduced in the mid 19th century and revolutionised photography. They cut down exposure time and thus allowed for some movement in the image without blurring as subjects were not required to sit for long periods to produce sharp results.