PSEITDOSCOPE (pseudos, false, and skopein, to see), an optical instrument invented by prof. Wheatstone, and so called from the fact of its exhibiting objects, viewed through it, under aspects the exact converse of their natural appearance. Its construction as shown by the annexed figure: abc, two rectangular prisms of flint-glass, the hypothenuses of which measure in. in length, and in. in depth. The prisms are hinged at c, so that they may be inclined toward each other in any desired degree, and are fixedeach in a wooden framework, fastened together by the hinge or pivot d. By means of the pivot mi, the distance between c and c may be made to accommodate itself to the different interval between the eyes in various observers. In using the instrument the eyes are placed at E and E respectively. The thumb-pieces attached to the prisms at a and a are used in adjusting the instrument for distinct vision of any particular object. The optical effect of each prism is twofold: it displaces the object, and it procures the lateral inver sion of the image. From the latter circumstance, it follows that the right-hand side of a cube, for example. is seen on the left, and vice versa; this in version being occasioned by the reflection of the rays upon the side of the hypothenuse within'each prism. By the two refractions undergone by the rays as they enter and leave the prisms, the axis of the emergent pencil is no longer directed toward the real place of the object, but is diverted in such sort that the convergence of the optic axes diminishes as the distance of the object diminishes; and increases as its distance increases—a complete reversal of the ordinary conditions of sight, and one which, in conjunction with the lateral inversion before mentioned, gives rise to very curious visual phenomena. Sec Vision It is essential to the efficient use
of this instrument that the object be seen by both eyes; and therefore the observer, haviug placed the object at the usual distance for distinct vision, should, by closing the eyes alternately, ascertain that it is within the field of each prism. He should then adjust the prisms until the two images coincide in point of space, when they will coa lesce, and, at first, the object will probably retain its natural aspect; but on a sudden it undergoes a change, and the converse appearance stands out to view with the utmost distinction and reality. A hat will appear to be turned completely inside out; the interior of a basin will appear convex and protruding; and "a bust regardod in front becomes a deep hollow mask." To facilitate the illusion, the object should he equally illuminated on either side, so as to prevent shadows.—For a full account see the original paper by Wheatstone, Phil. Trans., 1832, p. 11, et seq.