Source : Webster's Revised Unabridged Dictionary (1913)

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

   {Retinal purple} (Physiol. Chem.), the visual purple.

Source : WordNet®

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