THE MIND'S EYE. 



3°7 



but an irregular and meaningless assemblage of black and white 

 blotches. At a distance of fifteen to eighteen feet, however, a man's 

 head appears quite clearly. Also observe that after the head has 

 once been realized it becomes possible to obtain suggestions of it at 

 nearer distances. 



A much larger class of ambiguous diagrams consists of those 

 which represent by simple outlines familiar geometrical forms or 

 objects. We cultivate such a use of our eyes, as indeed of all our 

 faculties, as will on the whole lead to the most profitable results. As 

 a rule, the particular impression is not so important as what it repre- 

 sents. Sense impressions are simply the symbols or signs of things 



Fig. 12. — In this scroll the left half may at 

 first seem concave and the right convex, 

 it then seems to roll or advance like a 

 wave, and the left seems convex and the 

 right concave, as though the trough of 

 the wave had become the crest, and vice 

 versa. 



Fig. 11. — This represents an ordinary table- 

 glass, the bottom of the glass and the en- 

 tire rear side, except the upper portion, 

 being seen through the transparent nearer 

 side, and the rear apparently projecting 

 above the front. But it fluctuates in ap- 

 pearance between this and a view of the 

 glass in which the bottom is seen directly, 

 partly from underneath, the whole of the 

 rear side is seen through the transparent 

 front, and the front projects above the 

 back. 



or ideas, and the thing or the idea is more important than the sign. 

 Accordingly, we are accustomed to interpret lines, whenever we can, 

 as the representations of objects. We are well aware that the can- 

 vas or the etching or the photograph before us is a flat surface in 

 two dimensions, but we see the picture as the representation of solid 

 objects in three dimensions. This is the illusion of pictorial art. So 

 strong is this tendency to view lines as the symbols of things that if 

 there is the slightest chance of so viewing them, we invariably do so; 

 for we have a great deal of experience with things that present their 

 contours as lines, and very little with mere lines or surfaces. If we 

 view outlines only, without shading or perspective or anything to 

 definitely suggest what is foreground and what background, it be- 

 comes possible for the mind to supply these details and see fore- 

 ground as background, and vice versa. 



A good example to begin with is Fig. 8. These outlines will 



