Summary
The historical development of research which concerns the mesopic range is traced. Theories on the basic processes which initiate the visual response are outlined and consideration is given to the changes which accompany visual adaptation as well as the external influencing factors.
An attempt is then made to specify the mesopic range and the relation is drawn between this range and that over which gross changes in many visual functions occur as a result of a change of functional predominance. Although by far the most convenient demarkation is by reference to luminance level, it is realised that the transition range is also dependent on a number of factors such as the nature of the visual task with respect to the relative functions called for. The influence of retinal position is also stressed.
Various adaptation phenomena are investigated in an attempt to find evidence of relative functions. The only information from these experiments is inconclusive. The effect of using various wavelengths brings out factors of relative response by rods. Visual adaptation is reviewed and it is seen that visual acuity is poor at mesopic levels. Despite the significance of practical problems in this field, relatively little is understood and analysis of exact rod and cone function is difficult.
The examination of luminance threshold for resolution of gratings under various adaptation states provides clearer evidence for interaction, and emphasises the fallacies of simple division of duplex curves. Even here, however, quantitative proportions are not available. Next, the study of the Purkinje Shift produces one of the most promising fields for determining interaction and, although this field has little direct consequence, its applications are wide. Investigators agree that cone function can be exhibited at low mesopic levels and rod functions above cone threshold. Bridgman has made the first attempts to explain intermediate luminosity functions and has concluded that additivity is a significant factor.
Chromaticity functions have been examined, and it is found that in the mesopic range the eye changes from being tritanomalous at high levels to tritanopic at mid points and to monochromatic or rod vision at scotopic levels. Correlations have been found with luminosity functions.
The changes in refractive power of the eye are purely speculative in the mesopic range. The problems confronting vision at low illumination and twilight levels rely heavily on contrast effects. Vision at twilight is almost at its ebb.
It must be concluded that relatively little quantitative data on interaction has been accumulated, but it has been shown that the forces of interaction are complex in the mesopic range.
