The Collaborative International Dictionary of English v.0.48:

Retinal \Ret"i*nal\, a. (Anat.)
   Of or pertaining to the retina.
   [1913 Webster]

   Retinal purple (Physiol. Chem.), the visual purple.
      [1913 Webster]

WordNet (r) 3.0 (2006):

retinal purple
    n 1: a red photopigment in the retinal rods of vertebrates;
         dissociates into retinene by light [syn: visual purple,
         rhodopsin, retinal purple]