Vitamin A in the retina.

Through his research on xerophthalmia, karatomalacia and night blindness Wald revealed the critical function of vitamin A in normal vision and visual pathology.

(Thanks to Juan Weiss for this reference and their interpretation.)

In 1967 Wald shared the Nobel Prize in Physiology or Medicine with Ragnar Granit and Haldan Keffer Hartline "for their discoveries concerning the primary physiological and chemical visual processes in the eye."  See also Nos. 14276, 14277