Prof. Wheatstone on the Physioloyy of Vision. 253 



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 geo- 

 metrical 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 gene- 

 rally the case with regard to simple forms, a singular phenome- 

 non takes place ; it is perceived at one time distinctly as one of 

 these figures, at another time as the other, and while one figure con- 

 tinues 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 subject. It is by Professor Necker of Geneva, and 

 I shall quote it in his own words from the Philosophical Maga- 

 zine, Third Series, vol. i. p. 337. 



" The object I have now to call your attention to is an obser- 

 vation which has often occurred to me while examining figures 

 and engraved plates of crystalline forms ; I mean a sudden and 

 involuntary change in the apparent position of a crystal or solid 

 represented in an engraved figure. What I mean will be more 

 easily understood from the figure annexed (fig. 22). The rhom- 

 boid AX is drawn so that the solid angle A should be seen the 

 nearest to the spectator, and the solid angle X the farthest from 

 him, and that the face ACDH should be the foremost, while the 

 face XDC is behind. Put in looking repeatedly at the same 

 figure, you will perceive that at times the apparent position of 

 the rhomboid is so changed that the solid angle X will appear 

 the nearest, ami the solid angle A the farthest; and that the 

 face ACJ)H will recede behind the face XDC, which will come 



