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The prism binocular consists like the astronomical telescope of object lens and ocular. The astronomical telescope itself however delivers an upside down image; to reverse this image, one uses prisms. The second image shows, how a bundle of rays is reflected twice in an right angle by total reflexion, so that afterwards top and bottom are changed. Because on the upside down image in the telescope also left and right have been changed, the bundle of rays has to go through a second prism, so that the sides are correct reproduced. This constellation of prisms leeds to a essential shortened length in comparison to the terrestrial telescope, because of the threefold way of rays. The object lens distance, which is enlarged in comparison to the eye distance, is an advantage for three-dimensional view. On picture 4,one can see the place of intermediate image shortly before the ocular, where the bundle of rays comes together. Here one also can find the visual field blind, that we see as a sharp border of the image field in the infinity.