Stereoscope

Stereoscope (Gr. stereos, 'solid,' and skopein, 'to see'). Each eye of an observer forms its own retinal image of visible objects. These two images, being taken from slightly different points of view, are slightly different from one another, as may be seen by looking at near objects with each eye alternately; and they are the more so the nearer or the narrower the objects are. These two retinal images are blended by a process of interpretation of sensation, which interpretation is based on experience, into a mental image of the object seen as a solid object possessing three dimensions. Professor (afterwards Sir Charles) Wheatstone first pointed this out in 1838, and set himself the question, 'What would be the visual effect of simultaneously presenting to each eye instead of the object itself its projection on a plane surface as it appears to that eye?' He tried the experiment with drawings of cubes, &c., and found that when one eye was made to look at each drawing the two images blended into one which appeared to stand out in relief. Photography supplies more accurate representations of views from two points of view than the artist's eye and hand can supply; and if a view be taken by two lenses upon different parts of a single sensitive plate the print from the negative must be divided into two and the two pictures transposed and mounted. If this transposition be neglected the effect is pseudoscopic—i.e. instead of objects standing out in relief they stand back as if their more prominent surfaces were the walls of cavities. The stereoscope is essentially an instrument in which each picture is examined by a separate lens, and the two lenses are inclined so as to shift the images towards one another and thus to ensure or to facilitate the blending of the two images into one, besides which the lenses act as magnifying glasses. The two lenses must be equal. This may be ensured by using instead of whole lenses two halves of a single lens, the straight edges of which halves must be fixed parallel to one another.