Abstract
1. The rod branch of the dark-adaptation curve was studied at two regions 1(1/4) degrees from the foveal centre (see inset, Fig. 2).2. One region had been bleached by red light, the other by blue of equal scotopic value.3. This equality was established by finding the rod increment threshold against the red or blue backgrounds: they were judged equal when they raised the rod threshold equally.4. If the bleaching exposure lasted 4 min or less, the rod dark-adaptation curves coincided, though the cone branches did not (since red bleached cones 80%, blue 20%, but rods were bleached equally).5. If the bleaching exposure was prolonged to 16 min the rods recovered more slowly from the red bleach. And this difference was enhanced by a preliminary intense white exposure that bleached everything.6. The effect was due to the presence of cones since repeating the experiment at 15 degrees from the fovea where cones are scarce virtually gave coincident rod dark-adaptation curves.7. It follows that where cones are regenerating fast (from the 80% red bleach) they extract from the space outside the cone cells some ingredient needed for rod regeneration, and after pronounced and prolonged bleaching, the lack of this ingredient slows rod recovery.8. Most probably the ingredient is 11-cis retinol returning from the pigment epithelium.