256 



PSYCHOLOGY. 



two eyes, at the opening of a wine-glass or tumbler (Fig. 

 73), held either above or below the eye's level. The retinal 

 image of the opening is an oval, but we can see the oval in 

 either of two ways, — as if it were the perspective view of a 

 circle whose edge h were farther from us than its edge a 

 (in which case we should seem to be looking down on the 

 circle), or as if its edge a were the more distant edge (in 

 which case we should be looking up at it through the h side 

 of the glass). As the manner of seeing the edge changes, 

 the glass itself alters its form in space and looks straight 

 or seems bent towards or from the eye,* according as the 

 latter is placed beneath or above it. 



Plane diagrams also can be conceived as solids, and that 

 in more than one way. Figs. 74, 75, 76, for example, are am- 



FiG. 74. 



Fig. 75. 



Fig. 76. 



biguous perspective projections, and may each of them re- 

 mind us of two different natural objects. Whichever of these 

 * Cf. V. Egger, Revue Philos., xx. 488. 



