|Praxinotropes use mirrors around a central column to reflect the inside of the drum. The "overlapping" images create the sensation of movement.|
People like to be fooled. We find the notion of illusion entertaining, and go out of our way to believe in magic. Long before David Blaine, Imax theaters, and Florentines faffing about in velvet hats, there were much hairier creatures crawling around in caves, creating a visual magic of their own. 35,000 years ago, painters made their way into the belly of the earth and left behind a spectacular tableau of aurochs, horse, bison and elk, often composed of curious double images.
|The Chamber of Lions, in Lascaux, depicts curious double images, once believed to suggest the motion of their subjects.|
Archaeologists have long suggested that the double images are attempts to breath life into the stone; that under flickering torch light the animals are seen to dance and move. Early experiments in kinematics - or motion pictures - built on this principle by using a spinning barrel with staggered images on the inside that appear to move when spun, called a zoetrope or praxinotrope. Recently, however, a couple of artists provocatively suggested that these ghost-like double images are not illusions of movement but are in fact illusions of depth.*
|On the left, the camera is focused on my finger, and shows a blurred double image of the X on the wall.|
On the right, the camera is focused on the X, and the double image is instead my finger.
A simple experiment explains their idea: Hold your finger up in the air between your eyes and this page, and focus on it: You see one finger. Now shift your focus to the page behind it, and magically there appears to be two ghost images of your finger hovering. The double image of your finger is thus understood to be closer to you than the single image of the page. Simples!
Every child is familiar with this basic optical trick. It’s the optical displacement of an object caused by our binocular vision (or stereopsis), known as “parallax,” and it’s a binocular depth cue that babies learn as early as four months out of the womb. In order to navigate 3-dimensional space, we learn from a very early age to interpret our world via optical cues such as parallax, and organize it into "near" and "far," for example.
Perhaps our primitive ancestors were harnessing this basic navigational tool and using it to create images depicting not movement through space, but space itself. If so, they'd have beaten the linear perspective discoveries of Alberti by, oh, 34,000 years or so.
* [In a talk by Ryan and Trevor Oakes, titled “Seminal Notions: The Idea and Practice of Perspective,” given at the Chicago Humanities Festival.]