Purpose: Many eyes appear to have a dark ring at the limbus. The present work investigated whether this ring could be partly caused by optical properties of the eye near the limbus.

Methods: The eyes of 10 human subjects were immersed in water, by having subjects wear goggles filled with saline, to assess the changes in the limbal ring when the power of the cornea/air interface was largely eliminated. Eyes were photographed in air and in water, and images were analyzed to determine the prominence of the limbal ring. Corneal shape was determined for the same eyes using a commercial topography system.

Results: Immersion in water reduced the prominence of the limbal ring by approximately 55% on average.

Conclusions: The dark limbal ring has a substantial optical component in many eyes, although evidence was found for an anatomical basis in some eyes. Visibility of the ring may depend on extent of non-transparent cornea, which has implications for clinical estimation of angle width.