410 



PHYSIOLOGY 



CHAP. 



movements of the head or fixation - point are absent, we are 

 incapable of tri-dimensional vision. On looking, for instance, at 

 the drawing of a truncated four -sided pyramid (Fig. 194) the 

 immediate sensation is of a flat geometrical figure in two 

 dimensions. It requires a psychical act to recognise it as the 

 drawing of a body with three dimensions, when, that is to say, 

 we imagine that the small square which repre- 

 sents the truncated apex is nearer the eye than 

 the large square which represents the base of 

 the pyramid. This judgment is arbitrary, 

 because it is founded on no objective character 

 in the drawing. To so great an extent is this 

 true that we can with equal facility regard the 



FIG. 194. Diagram of same figure as a hollow body in pyramidal form, 

 qu U adran e guF a y r r bai dwith ^7 imagining that the large square is closer to 

 our eye than the small square. In the same 

 way the diagram of Fig. 195 can be interpreted either as a ladder 

 placed against a wall, or as the under surface of a staircase built 

 on to a wall. In the first case we imagine the right angle a to 

 be nearer our eye than the angle b ; and vice versa in the second 

 case. Both judgments are arbitrary, because in such a figure there 

 are none of the accidental features of light 

 and shade by which, on looking at a draw- 

 ing, painting, or photograph, we obtain the 

 idea of relief, or three-dimensional form. 

 On looking, e.g., at Fig. 196 no one would 

 hesitate in deciding that it was a pyramid 

 with a hexHgonal base, as the spire on a 

 church tower; but we are unable to say 

 from objective data whether the flag on the 

 summit of the pyramid is turned towards 

 or away from us, because there is no physio- 

 logical basis for either judgment. 



Uniocular people, who are forced always 

 to use one eye, develop the faculty of uni- 

 ocular tri-diniensional vision of external 

 objects to a remarkable extent, as the blind 

 do the spatial tactile perceptions. In fact, 

 when they learn to draw and paint they 

 are well able to reproduce the physical appearance of objects. 

 Guercino's pictures are remarkable for the relief in which the 

 figures stand out from the background ; yet they were executed 

 by the famous painter with only one eye. 



None the less it is certain that the perception of the three 

 dimensions is normally much more certain, complete, and direct 

 with binocular vision. Of this we have a simple and very 

 convincing proof in the fact that many people cannot thread a 



FIG. 195. Schroder's staircase. 



