MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 381 



would do were it substituted foi* it. The appearance which M'ould make us certain 

 that it is a picture is excluded from the sight, and the imagination has room to be 

 active. Several of the older writers erroneously attributed this apparent superiority 

 of monocular vision to the concentration of the visual power in a single eye *. 



There is a well-known and very striking illusion of perspective which deserves a 

 passing remark, because the reason of the effect does not appear to be generally un- 

 derstood. When a perspective of a building is projected on a horizontal plane, so 

 that the point of sight is in a line greatly inclined towards the plane, the building ap- 

 pears to a single eye placed at the point of sight to be in bold relief, and the illusion 

 is almost as perfect as in the binocular experiments described in §^ 2, 3, 4. This 

 effect wholly arises from the unusual projection, which suggests to the mind more 

 readily the object itself than the drawing of it; for we are accustomed to see real 

 objects in almost every point of view, but perspective representations being generally 

 made in a vertical plane with the point of sight in a line perpendicular to the plane 

 of projection, we are less familiar with the appearance of other projections. Any 

 other unusual projection will produce the same effect. 



/ 



§ 10. / 



If we look with a single eye at the drawing of a solid geometrical figure, it may 

 be imagined to be the representation of either of two dissimilar solid figures, the figure 

 intended to be represented, or its converse figure (§ 5.). If the former is a very usual, 

 and the latter a very unusual figure, the imagination will fix itself on the original 

 without wandering to the converse figure ; but if both are of ordinary occurrence, 

 which is generally the case with regard to simple forms, a singular phenomenon 

 takes place; it is perceived at one time distinctly as one of these figures, at another 

 time as the other,and while one figure continues it is not in the power of the will to 

 change it immediately. 



The same phenomenon takes place, though less decidedly, when the drawing is 

 seen with both eyes. Many of my readers will call to mind the puzzling effect of 

 some of the diagrams annexed to the problems of the eleventh book of Euclid ; which, 

 when they were attentively looked at, changed in an arbitrary manner from one solid 

 figure to another, and would obstinately continue to present the converse figures 

 when the real figures alone were wanted. This perplexing illusion must be of com- 

 mon occurrence, but I have only found one recorded observation relating to the sub- 

 ject. It is by Professor Necker of Geneva, and I shall quote it in his own words 

 from the Philosophical Magazine, Third Series, vol. i. p. 337- 



" The object I have now to call your attention to is an observation which has often 



* " We see more exquisitely with one eye shut than with both, because the vital spirits thus unite them- 

 selves the more, and become the stronger : for we may find by looking in a glass whilst we shut one eye, 

 that the pupil of the other dilates."— Lord Bacon's Works, Sylva Sylvarum, art. Vision. 



