250 Prof. Wheatstone on the Physiology of Vision. 



fig. 15, a and b are symmetrical monocular projections of the 

 frustum of a four-sided pyramid, and figs. 13, 14, 16, are cor- 

 responding projections of other symmetrical objects. This being 

 kept in view, I will describe an experiment which, had it been 

 casually observed previous to the knowledge of the principles 

 developed in this paper, would have appeared an inexplicable 

 optical illusion. 



M and M' (fig. 21) are two mirrors, inclined so that their 

 faces form an angle of 90° with each other. Between them in 

 the bisecting plane is placed a plane outline figure, such as 

 fig. 15 a, made of card all parts but the lines being cut away, or 

 of wire. A reflected image of this outline, placed at A, will 

 appear behind each mirror at B and B', and one of these images 

 will be the inversion of the other. If the eyes be made to con- 

 verge at C, it is obvious that these two reflected images will fall 

 on corresponding parts of the two retina?, and a figure of three 

 dimensions will be perceived; if the outline placed in the bisect- 

 ing plane be reversed, the converse skeleton form will appear ; 

 in both these experiments we have the singular phsenomenon of 

 the conversion of a single plane outline into a figure of three 

 dimensions. To render the binocular object more distinct, con- 

 cave lenses may be applied to the eyes ; and to prevent the two 

 lateral images from being seen, screens may be placed at D 

 and D'. 



§8. 



An effect of binocular perspective may be remarked in a 

 plate of metal, the surface of which has been made smooth 

 by turning it in a lathe. When a single candle is brought 

 near such a plate, a line of light appears standing out from 

 it, one half being above, and the other half below the surface ; 

 the position and inclination of this line changes with the 

 situation of the light and of the observer, but it always passes 

 through the centre of the plate. On closing the left eye the 

 relief disappears, and the luminous line coincides with one of 

 the diameters of the plate ; on closing the right eye the line 

 appears equally in the plane of the surface, but coincides with 

 another diameter; on opening both eyes it instantly starts into 

 relief*. The case here is exactly analogous to the visiou of two 

 inclined lines (fig. 10) when each is presented to a different eye 

 in the stereoscope. It is curious, that an effect like this, which 

 must have been seen thousands of times, should never have 



* The luminous line seen by a single eye arises from the reflexion of 

 the light from each of the concentric circles produced in the operation of 

 turning ; when the plate is not large the arrangement of these successive 

 reflexions does not differ from a straight line. 



