The blue arcs of the retina.

Around a dim light viewed in a dark room can be seen faint blue-gray arcs which occupy that part of the visual field corresponding to the retina where the arcuate nerve fiber bundle passes from macular ganglion cell bodies to the optic nerve. These blue arcs of the retina are an entoptic phenomenon in which action potentials of the arcuate nerve fiber bundle presumably excite adjacent neurons. The experiments here described show that the light stimulus initially evoking the blue arcs excites cones and not rods as has been generally believed until now. Another commonly held idea is that the blue arcs are produced by bioluminescence or fluorescence associated with the action potentials in the arcuate nerve fiber bundle. The experiments described here disprove this hypothesis.