VISION 



401 



they are determined by the size of the image on the retina. 

 This observation is most surprising if made one or two nights 

 after full moon, when twilight is already dim at moon-rise. 



Our estimate of the distance away from us of an object on 

 the horizon is based upon the time and effort which experience 

 tells us we should need to spend in reaching it. The untried 

 appears shorter than the tried. Anyone who compares his 

 feeling of the number of yards he would have to climb up a pole 

 reaching to the zenith with his feeling of the number of steps 

 he would need to take to reach the horizon will recognize that 

 the horizon appears to him to be the farther away. 



FIG. 36. A SYMMETRICAL ARCH, DIVIDED BY A VERTICAL LINE, A, WHICH PASSES THROUGH 



ITS APEX. 



In representing a solid object an artist conveys the idea that 

 light is falling obliquely upon it. One side of the object, 

 therefore, is more strongly illuminated than the other. By 

 depth and gradation of shade he indicates the extent to which 

 the thing projects forwards, if solid, or falls back, if hollow, 

 He makes the margin of a ball hazy, in the expectation that 

 the spectator will look at the spot nearest to him an artifice 

 which he may easily press too far, since the eyes wander rest- 

 lessly over a flat surface. In representing distance he is 

 dependent upon giving to the various objects in his picture 

 sizes equivalent to the sizes of their images on the retina, 

 making them brighter or paler and more or less distinct. Yet 

 he cannot hope to simulate the convincing evidence of distance 

 which is afforded by our sense of the degree of convergence of 



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