A prehminary difficulty in all theories of vision is the fact that physicists can give us very little satisfactory information as to the real nature of light. All the chief theories of vision are based upon the existence in the eye of certain hypothetical substances whose decomposition under the influence of entering light-rays, is conjectured to produce the necessary effect upon the end-organs of the retina, namely the rods and cones. Naturally a vast amount of experimentation has been made upon visual sensations and many ingenious methods. Neurologists have discovered that differences in stimulus qualities are probably communicated to the brain by different rates, rhythms, or volumes of afferent nerve impulses. These quantitative differences in nerve impulses seem to be originated and determined by the appropriate sensory receptor organs with which the afferent nerves connect.