THE STEREOSCOPE : ITS HISTORY. 



43 



now classic paper on the " Physiology of Vision." Let the reader im- 

 agine, 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 succes- 

 sion 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 dis- 

 similar pictures projected by it on the two retinse, the following ques- 

 tion occurs : What would be the visual effect of simultaneously pre- 

 senting 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 retinse. Under the ordinary circumstances of vision, the object 

 is seen at the concourse of the optic axes (visual lines *), and its im- 

 ages consequently are projected on similar parts of the two retina? ; 

 but it is also evident that two exactly similar objects may be made to 

 fall on similar parts of the two retince, if they are placed one in the 

 direction of each optic axis, at equal distances before or beyond their 

 intersection." 



Fig. 4. Wheatstone's Stereoscope (Perspective View), 1833. 



To follow out to the letter the instructions suggested in Wheat- 

 stone'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 var- 

 nish ; 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. 



