What visual aid did Leonardo da Vinci propose placing directly on the eye?

Leonardo da Vinci proposed a contact lens concept that would place a water-filled device directly against the eye.

In his Codex of the Eye, Leonardo explored how vision works and suggested immersing the eye in water inside a glass vessel. The water and curved surface would alter the path of light entering the eye, anticipating the optical principle behind contact lenses.

His proposal was not a practical contact lens in the modern sense. It did not provide a wearable, comfortable, medically safe device, and Leonardo did not manufacture one. The idea was part of an optical thought experiment connected with his study of the eye, light, and image formation.

Modern contact lenses are thin, shaped lenses placed on the tear film covering the cornea. They are made from specialized plastics or silicone-based materials and require careful fitting and hygiene. Leonardo’s proposal is therefore best understood as an early optical concept rather than the invention of a usable lens.