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It is now well established that light rays of the same spectral character and physical intensity entering the eye through different points of the pupil may produce visual impressions which differ in brightness and colour even though the patch of retina stimulated (the fovea) is kept the same. Rays entering the eye through different points of the pupil and terminating on the same point of the retina are incident on the retina in different directions. Also, they have traversed different paths in the refractive media of the eye and may have suffered different losses by absorption, scattering or reflexion. It has been shown, however, that differences in the light losses in the refractive media do not account for the observed variations in visual response, which must therefore be attributed to variations in the reaction of the retina to light incident on it in different directions or, briefly, to a directional sensitivity of the retina. Further evidence that this is so is given later in the paper (p. 81).

The directional sensitivity of the retina and the spectral sensitivities of the rods and cones