And dizzy ’tis to cast one’s eyes so low!

The illusion of the third dimension is discussed at length in E. H. 
Gombrich’s Art and Illusion.  Far from being a normal mode of 
human vision, three-dimensional perspective is a conventionally 
acquired mode of seeing, as much acquired as is the means of 
recognizing the letters of the alphabet, or of following 
chronological narrative. That it was an acquired illusion 
Shakespeare helps us to see by his comments on the other 
senses in relation to sight. Gloucester is ripe for illusion because 
he has suddenly lost his sight. His power of visualization is now 
quite separate from his other senses. And it is the sense of 
sight in deliberate isolation from the other senses that confers 
on man the illusion of the third dimension, as Shakespeare 
makes explicit here. There is also the need to fix the gaze: