An important modification of Wheatstone's stereo scope has just been communicated to the Academy of Sciences by M. d f Almeida. With the cdiriraon instru ment only one observer at A time can view the relief j M. d'Ahneida renders it visible to several at a time and at a distance of ae\ oral metres. For this purpose he causes two stereoscopic images to be reflected simul taneously on a screen ; as they are not identical, but only similar, the outlines of the one will intersect those of the other, and generate a confusion which can only be obviated by making each eye see only one of the images. For this purpose the inventor causes the luminous rays from each image to pass through a glass of a different colour — one red and the other greer \ whereby one of the images will be reflected on the screen in red» the other in green. Now, if the observer's eyes be provided with glasses of the above-mentioned co lours, the eye covered with a green glass will only see the green image, while the other will only be visible lo the eye protected by a red glass. The moment this is effected the relief appears, and if tile observer shift his position laterally, *he figure -will appear to move in A contrary direction, which adds to the illusion. M. d' Almeida proposes another plan, in which both images are uncoloured, and each eye is made to perceive one image only by rapidly intercepting the other from view by means of a revolving piece of pasteboard, cut so as only to cover one of the images at a time at each half-re volution. As soon as the rotatory motion acquires sufficient rapidity the figures appear m relief.
