SPECTRAL SENSrTVITY CURVES AND THE ABSORPTION OF LIGHT BY THE OCULAR MEDIA* BY

THE inter-relation of sensory, electrophysiological, and photochemical data in vision requires a knowledge of the behaviour of the retina -in the absence of the ocular media. As far as the human eye is concerned, such knowledge is very inadequate in the case of the enucleated eye, and non-existent in that of the eye in situ. While light absorption by the pre-retinal and intraretinal media cannot affect any mode of colour-vision, it modifies the sensitivity to light. The sensitivity of the eye to radiations of different wavelengths is usually defined as the reciprocal of the spectral energy required to produce a given constant sensation. It is immaterial whether the constancy means equality of brightness, threshold of vision, or absence of flicker; in all instances the experiment involves the measurement of energy. Since the photo-receptors are adjacent to the pigment epithelium, light reaching them must have travelled through the cornea, aqueous, lens, vitreous, and a major part of the retina proper. Every one of the structures acts as a light filter, thus reducing its intensity, and necessitating a larger supply of energy for the production of a given sensation than would otherwise be required. The energy is measured at the cornea. It follows that the sensitivity as measured at the cornea, Se, is less than the receptoral sensitivity SREC (Thomson, 1951). If the pre-receptoral structures were neutral, i.e., if the percentage absorption of light were the same for all wavelengths, a knowledge of their characteristics would be important only in connection with measurements of the absolute sensitivity. All the scanty available evidence, however, indicates that they absorb selectively in different spectral regions (see Ludvigh and McCarthy, 1938, for data on human and animal eyes), the predominant absorption taking place at shorter wavelengths. The unequivocal measurement of all the absorption characteristics in an intact human eye in situ is impossible. If enucleated eyes are used (Ludvigh and McCarthy, 1938), autopsy may mask the results, especially if the eyes are kept at 380 C., as should be done in all such cases. The application of animal data to human results is a doubtful remedy: in any case the intraretinal absorption cannot be determined in this or in any other manner. This depressing outlook is somewhat mitigated by the circumstance that the spectral absorption of the pre-retinal media at long waves (green to red parts