THE SPECTRAL SENSIBILITY OF THE SUN-FISH AS EVIDENCE FOR A DOUBLE VISUAL SYSTEM

1. An extension of a previously described method makes possible the measurement of the visibility function of Lepomis at high intensities of spectral illumination. This is accomplished by determining the relative energies of various spectral beams which will just produce a visual orienting response by the animal to the movement of a pattern composed of fine lines. 2. The function so determined is different from that obtained with a pattern composed of wide bars and spaces at a lower intensity level. 3. This difference furnishes direct and quantitative proof that the eye of Lepomis is a physiologically duplex visual system and parallels the known anatomical distinctions between the rods and cones. 4. A comparison of the visibility curves of the two systems indicates that both functions are similar in shape but that the cone curve is shifted to the red. 5. It is suggested that this relation between the two systems, which is also found in the human and the fowl, indicates that the photosensory substance is the same in each case for the rods and cones. According to this hypothesis, the shift of the cone curve is due to a common physical cause which depends on differences in the properties of the solvent media in the cones and in the rods.