What material did Thomas Young use to affix water-filled lenses to his eyes?

Thomas Young used wax to affix water-filled lenses to his eyes in his early experiments with contact lenses. This choice is significant because wax has adhesive properties that can help in holding the lenses securely in place while being non-irritating to the eye. Young's experimentation with contact lenses was pivotal in the understanding of how lenses can be used to correct vision.

Other choices, such as silica, epoxy, and saliva, were not practical or commonly used materials for this application in early lens experiments. While saliva might offer some moisture, it does not provide a stable adhesive bond needed for secure lens placement. Silica and epoxy would not only lack the necessary properties for safe ocular application, but they could also pose certain risks if used directly in contact with the eye. Thus, wax stands out as a historically significant and appropriate material used by Young in his pioneering studies of lenses.