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W. JAMES I 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 I 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 & 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, 1 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. 19, 20, 21, for example, are am- biguous perspective projections, and may each of them remind Fig. 20. 19. Fig. 21. us of two different natural objects. Whichever of these objects we conceive clearly at the moment of looking at the figure, we seem to see in all its solidity before us. A little practice will enable us to flap the figures, so to speak, backwards and forwards from one object to the other at will. 1 Cp. V. Egger, Revue Philos., xx. 488.