VISION AND THE TECHNIQUE OF ART. 15 



dark objects are narrow enough, appear very bluish while the lights 

 next to the dark objects appear pale orange. Of course if the focus of 

 the eye is not kept fixed on the central object these appearances will 

 not be visible for they only exist on objects that are not in focus. It 

 may take some practice to keep from changing the focus as the natural 

 tendency is to focus upon the object which one is trying to analyze. 



The colored edges on both far and near objects are best seen when 

 the objects are at the relative distances above described. Red edges 

 on the dark become more evident with a more distant fixation and 

 blue on the dark with a nearer one. 



Where both the fixation and the nearer object are twenty feet or 

 more away both objects are so nearly in the same focus that the 

 difference in colored edges is hard to distinguish. 



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

 tion the characteristic colored edges of objects nearer and farther than 

 the fixation object inform us of their relative distance, that is, if an 

 object has red edges we judge it to be nearer to us than the fixation 

 object while if it has blue objects we judge it to be farther away. If 

 this is so objects depicted in a picture with red edges should appear 

 nearer than those with blue edges. This is exactly what was found to 

 be the case as is shown by Figure 17a. It will be seen that the circles 

 with the reddish edges appear to be on a nearer plane than those with 

 the blue edges. 5 



SUMMARY. 



The effect of depth of focus which produces the fuzzy edges of objects 

 nearer or farther than the object plane has been long recognized in 

 photography. 



This is shown in the photographs in Figures 18 and 19. That in 

 Figure 18 was taken with a small aperture, F 16, which causes objects 

 at all distances to be imaged sharply, while that in Figure 19 was taken 

 with a large aperture, F 4.5, which causes objects nearer and farther 

 than the focus point to the imaged with a greater softening of edge. 

 The greater effect of depth in the photograph in Figure 19 as compared 

 with that in Figure 18 is very evident. 



Gleichen 6 goes into the subject at length and points out that a 



5 The illusion is more apparent if one eye is closed. This prevents the 

 functioning of our binocular depth perception by which we tend to recognize 

 the true distance of both figures. 



6 "Die Grundgesetze der naturgetreuen photographischen Abbildung." 

 Halle 1910. "Uber Helligkeit und Tiefe inbesondere bei der naturgetreuen 

 Photographischen Abbildung." Zeit, fur Wiss. Phot, Vol. 9, 1911, p. 241. 



