VISION AND THE TECHNIQUE OF ART. 23 



cross near the edge. If a distant object is focused the vertical line 

 in the large cross is more evident than the horizontal line. If the 

 little cross on the edge of the page is fixated the opposite is the case. 

 Care must be taken not to let the accommodation or line of vision vary. 

 These facts lead naturally to the assumption that with a given fixa- 

 tion the characteristic imaging of oblique objects in space informs us of 

 their distance; that is if an object has sharp tangential edges we judge 

 it to be nearer than an object with soft tangential edges and if it has 

 sharp radial edges we judge it to be farther away than an object with 

 soft radial edges. If this is so, objects depicted in a picture with sharp 

 tangential edges should appear nearer than those depicted with soft 

 ones and objects depicted with sharp radial edges should appear 



Figure 30. Figure for observing change of appearance of radial and 

 tangential lines with variation of focus. 



farther away than objects depicted with soft ones. This is exactly 

 what was found to be the case as is shown by Figures 30a and 30b. 

 Carefully observing 30a it will be seen that the circles with the sharp 

 edges, whether inside or outside appear nearer than the circles with the 

 soft edges. In Figure 30b it will be seen that the central portions of 

 figure I which are sharp edged appear to be more distant than the 

 circumference where the edges are soft, while in figure II where the 

 radial edges near the center are soft and those near the outside sharp 

 the center seems to be on the same plane as or nearer than the cir- 

 cumference. 8 



8 These illusions are more apparent if one eye is closed for the reasons given 

 in the footnote page 15. 



The judgment of two hundred and two students at Dartmouth College were 

 obtained on the effect of these illusions. On the figures in 30a 77.3% 

 judged that the circles with the sharp edges appeared nearer than those with 

 the soft ones. 19.8% made the opposite judgment and 2.9% got no effect. 



On the figures in 30b, I and II, 82.2% judged that the central portions of 

 Fig. I appeared more concave than that in Fig. II. 17.8% made the opposite 

 judgment. 



