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Ives stereoscopic Kromscop viewer, c 1890.
By means of this device, patented by Frederick Eugene Ives (1856-1937) in 1890, three stereoscopic positives can be viewed exactly superimposed. It makes use of the principle, discovered by James Clerk Maxwell in the 1860s, that all colours are a mixture of red, green and blue light. Each of the positives is either a red, green or blue record made from a negative taken through the appropriate filter. The positives are placed over corresponding filters in the viewer. Mirrors inside the box reflect the images towards the viewing lenses. The effect is the creation of an apparently three-dimensional colour image. The stereoscope was first invented by Charles Wheatstone who used it to demonstrate binocular vision to the Royal Society in 1838.