The spectral sensitivity of the retina has been measured by finding the energy required at various wave-lengths through the visible spectrum to match the brightness of a fixed comparison field. Results are recorded for two observers for a series of brightness levels within the Purkinje range and for three retinal areas, the fovea, 3 degrees from the fovea and 10 degrees from the fovea. A small shift of the foveal luminosity curve towards shorter wave-lengths has been observed with decreased brightness of the matching field, accompanied by the development of a hump on the red side of the curve. This may be due to the progressive isolation of the red response at low intensities. For the extrafoveal areas, the usual Purkinje shift was observed, although surprisingly high intensities were required before the photopic curve was approached. This suggests that the rods may continue to function over a greater range of intensities than is generally accepted. No sudden change from rod to cone vision was recorded, in contrast to other visual functions, and of the possible mechanisms by which the transition from one system to the other may operate, the special tendency of the rod response towards convergence and summation at low intensities seems the most likely. Approximate solutions to problems in photometry concerned with the change in relative brightness of coloured surfaces with reduced illumination may be derived by comparison with the corresponding changes which occur between monochromatic radiations of the same hue.