PROF. JOLY'S papers on vision are very interesting. He adopts the visual purple as the visual substance, but there is no evidence that the rods are percipient elements. The view that they are percipient elements is based on errors, as, for instance, that certain animals—the tortoise is the most quoted—possess only cones; that the periphery of the retina is colour-blind; and that the Purkinje phenomenon is not found with the fovea. The tortoise has the rods and cones as definitely marked and distinct from each other as in man. Has any reader seen a retina in which there are only rods or only cones in any animal? The periphery of the retina is not colour-blind. Red of sufficient luminosity can be seen to the extreme periphery. The Purkinje phenomenon is found with the fovea, and is a photochemical phenomenon. It is very improbable that the rods are percipient elements. An elaborate nervous mechanism is required to regulate the sensitiveness of the photochemical film, and this appears to be the function of the rods.