Stereoscope, after David Brewster (1849)

By means of a stereoscope you can see two plane (two-dimensional) images as one image. This creates the illusion of a spatial, three-dimensional image. The first stereoscope was devised by the English physicist Sir Charles Wheatstone in 1832 [See 331]. It is based on the principle that each eye sees a slightly different image. When we look at an object, the left eye sees it from a different perspective than the right eye. This difference in observation is the reason why we can judge distances and observe depth and relief. The first stereoscope worked by means of two mirrors, which reflected two images, specially made for this purpose, as one image to the eyes. In 1849 David Brewster devised a variant with two lenses. In order not to disturb the illusion, it was necessary that the lenses gave exactly the same deviation to the light beams. Therefore, Brewster made the two lenses out of one double-convex lens.