Why Did Leonardo Believe the Eye Saw the World Upside Down?

Image: Leonardo da Vinci, Public domain, via Wikimedia Commons

Leonardo da Vinci believed the eye was one of the most important organs in the human body. As both a painter and a scientist, he relied on vision to study light, movement, distance, and the natural world. His interest in the eye began in his earliest surviving notebooks and continued for much of his life. At one point, he even planned to write a complete treatise explaining how human vision worked (Ackerman, 1978).

Leonardo’s investigations brought him surprisingly close to the modern explanation of sight. However, one problem prevented him from reaching the correct conclusion.

So what did Leonardo believe happened inside the eye?

Leonardo compared the eye to a camera obscura, a darkened room or box with a small opening on one side. When light passes through the opening, an image of the outside world appears on a surface inside. The image is inverted because the rays of light cross as they pass through the small hole. Light from the top of an object reaches the bottom of the projected image, while light from the bottom reaches the top.

The basic principle of the camera obscura had been known long before Leonardo. The medieval scholar Alhazen discussed the device and understood that the image passing through the opening would be inverted. Leonardo also studied the camera obscura repeatedly and appears to have understood that the pupil worked in a similar way, allowing light to enter the eye and cross inside it (Ackerman, 1978).

This created a difficult question. If light formed an upside-down image inside the eye, why did the world appear right-side up?

Leonardo could not accept that an inverted image might be transmitted to the brain and interpreted correctly. In this respect, he was not unusual. According to art historian James Ackerman, scholars before Johannes Kepler generally rejected the possibility that the brain received an upside-down image.

Leonardo, therefore searched for another explanation. He believed that light passed through a transparent, spherical structure inside the eye. According to his theory, this sphere turned the image a second time before it reached the part of the brain responsible for vision. The second reversal would allow the viewer to perceive the world in its proper position (Ackerman, 1978).

The explanation was creative, but it was incorrect. The image formed inside the eye does remain inverted. The brain processes the visual information rather than depending on a second structure to turn the image upright.

Leonardo never completed a fully accurate theory of vision. Even so, his studies reveal the way he approached scientific problems. He read the work of earlier scholars, compared their claims with his own observations, and then attempted to create a new explanation. Sometimes those explanations worked. In this case, Leonardo came remarkably close to the truth but could not accept what his own experiment seemed to reveal.

Further Reading

Ackerman, J. S. (1978). Leonardo’s eye. Journal of the Warburg and Courtauld Institutes, 41, 108–146.