The Absorption Spectrum of Visual Purple by Aurin M. Chase and Charles Haig

The classical absorption spectrum of visual purple (KSttgen and Abelsdorff, 1896) is the difference in photometric density (log Io/I) at different wave-lengths between a solution in its unbleached and its bleached condition. The reason for this procedure is that extracts of visual purple are not pure but contain other substances which may be colored. The resulting classical subtraction curve is a nearly symmetrical absorption spectrum with a maximum at about 500 m/z. I t undoubtedly represents an important property of visual purple, because it resembles closely the rod visibility curve (Hecht and Williams, 1922; Weaver, 1937; Dartnall and Goodeve, 1937). The subtraction procedure is valid only if visual purple is the one light-sensitive substance in the retinal extract, if no new color appears when visual purple is bleached, and if the absorption of the photolabile group in the molecule is solely responsible for its color. I t is not certain that visual purple is the only light-sensitive substance in the frog's retinal extract (Chase, 1937), and it has been repeatedly observed that under some conditions a new color arises when visual purple is bleached (Kiihne, 1879; Garten, 1907; Nakashlma, 1929; Weigert and Nakashima, 1930; Hosoya, 1933; Wald, 1935; Chase, 1936; Lythgoe, 1937; and others). Finally, there is no experimental justification for the assumption that the molecule aside from its light-sensitive group or groups is colorless. The protein nature of the molecule (Kiihne, 1879; Wald, 1935; and Hecht, Chase, and Shlaer, 1937) makes the third assumption even less tenable because proteins absorb some light in the visible region of the spectrum. If the solutions could be freed of other colored substances, then the