now classic paper on the "Physiology of Vision." Let the reader imagine, or actually put on the page before him, some small solid body, such as a cone, with a few lines drawn from its vertex to the base. If it be of glass, so much the better; an ink-dot can then be marked at the center of the base, and the lines scratched upon the sides can easily be blackened. Close the left eye; the cone appears to the right eye like Fig. 1, R. Without moving the head, look with the left eye alone; the appearance is like Fig. 1, L. If each eye were in succession transformed for a moment into an electric light, the shadows projected upon the paper would be those given in the figure, but with a common base. Opening both eyes, the perception of the height of the cone is far more distinct than when either is closed. Let us now quote Wheatstone's own words: "It being thus established that the mind perceives an object of three dimensions by means of the two dissimilar pictures projected by it on the two retinæ, the following question occurs: 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? To pursue this inquiry, it is necessary that means should be contrived to make the two pictures, which must necessarily occupy different places, fall on similar parts of both retinæ. Under the ordinary circumstances of vision, the object is seen at the concourse of the optic axes (visual lines), and its images consequently are projected on similar parts of the two retinæ; but it is also evident that two exactly similar objects may be made to fall on similar parts of the two retinæ, if they are placed one in the direction of each optic axis, at equal distances before or beyond their intersection."
To follow out to the letter the instructions suggested in Wheatstone's last sentence, transfer Fig. 1 to glass. This can be easily done. Upon an oblong plate of window-glass put a few drops of clear varnish; let it spread thinly over the surface and become thoroughly dry.
- In Wheatstone's time the visual lines were supposed to be optic axes. That this is not quite so has since been proved by Helmholtz.