376 STEREOSCOPE nia tubular bridge across the Menai straits. (See BRIDGE, vol. iii., p. 275.) He was also employed on railways in Belgium, Sweden, Norway, Italy, France, and other parts -of Europe, and visited Egypt several times to superintend the construction of a road be- tween Alexandria and Cairo, on the line of which are two tubular bridges, traversed by trains on the roof instead of the inside, as in the case of the Britannia bridge. He also designed an immense bridge across the Nile at Kaffre Azzayat, and the great Victoria tu- bular bridge which crosses the St. Lawrence near Montreal, and was formally opened in the summer of 1860. From 1847 till his death he represented the Yorkshire borough of Whitby in parliament. He was a member of several scientific bodies, received a great gold medal of honor from the French industrial exposition of 1855, and from 1855 to 1858 was president of the institute of civil engineers. He pub- lished " Description of the Locomotive Steam Engine" (4to, London, 1838); "Report on the Atmospheric Rail way System " (4to, 1844) ; and "The Great Exhibition, its Palace and Contents" (12mo, 1851). Besides Smiles's biography, his life has been written by J. C. Jeaffreson and W. Pole (2 vols., London, 1864). STEREOSCOPE (Gr. areped^ solid, and CT/COTT&V, to see), an instrument by aid of which the two eyes view two different pictures of the same object and combine them into one having the appearance of solidity. This illusion is pro- duced by presenting to the right eye a picture which represents the object in perspective as it would appear to that eye alone, and to the left eye the picture of the object as seen by the left eye. If these two pictures exactly represent the object as seen respectively by the right and the left eye, which can readily be accomplished by means of photography, we shall, on looking into the stereoscope, re- ceive the same impression of solidity or relief as is given when both eyes look at the real object. One who has sufficient power of di- recting the movements of his eyes does not need an instrument to aid him in combining the two pictures on a stereoscopic slide. It is only required that the right eye and the left shall be respectively directed to corre- sponding points on the right-hand and left- hand pictures. It is said that a stereoscope as just described was conceived by Prof. El- liot of Edinburgh in 1834, but was not con- structed by him till 1839, after Sir Charles Wheatstone had in 1838 invented and exhibit- ed his reflecting stereoscope. In Wheatston-e's instrument the observer looks with his left eye into a mirror at a, fig. 1, and with his right eye into a mirror at ft. These mirrors are inclined at an angle of about 45, and hence reflect into the eyes the two pictures placed at k and g. These pictures therefore appear at the same place behind the two mirrors, and give the observer the impression that he is looking at an object or group of objects having solidity, or the third dimension. In 1849 Sir David Brewster invented a refracting stereoscope. This is more convenient than Wheatstone's, but does not give such well defined effects as the reflecting instrument. In Brewster's ste- FIQ. 1. Wheatstone's Stereoscope. reoscope the two picture^ are placed side by side, and are separated from each other by a partition, S, fig. 2, so that the right eye can only view the right-hand picture and the left eye the left-hand one. These two pictures are FIG. 2. firewater's Stereoscope. observed through two lenticular prisms, L and R, fig. 3, which not only slightly magnify the pictures, but also cause them to overlap each other; and thus we see in the middle of the instrument one picture which appears in relief. FIG. 8. Section of the Eye Pieces. These effects of solidity can readily be exag- gerated by taking the two pictures by means of two photographic cameras, the distance be- tween the centres of whose lenses is greater than that between the centres of the human eyes. The explanation of the illusions of the stereoscope is contained in the explanation of the fact that binocular vision gives us the per- ception of the third dimension of extension in all objects not over 200 ft. distant from the eyes ; for in the stereoscope we have the ima- ges formed on the retina of the right eye and of the left similar to the images that would be formed in the eyes if real solid objects were before us, having the sizes and the situations
