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1. Theoretical equations which predict the electrical response of turtle cones to a wide range of light stimuli are developed from the experiments described in previous papers.

2. The central points in the theory are that (a) light starts a chain of reactions leading to the production of a substance which blocks ionic channels in the outer segment, (b) an equilibrium between blocking molecules and open channels is rapidly established, (c) the blocking molecules are removed or inactivated by a chain of reactions, the first of which is autocatalytic, (d) in addition to the conductance which decreases with light there is also a conductance which increases with a delay when the cone is hyperpolarized.

3. Parameters in the theory were deduced by approximate equations from the experiments described in the previous papers.

4. There was good agreement between the properties of real and model cones in the following cases: (a) the response to 10 msec flashes and 0·7 sec steps of light calculated to give between 20 and 5 x 10⁷ photoisomerizations per cone at times extending to about 2 sec; (b) the complicated changes in the response to a test flash that occur when it is superposed on background lights of increasing intensity; (c) the after-hyperpolarization and period of reduced sensitivity following a strong flash.

5. The main defect of the theory is that the effect of background light in shortening the time to maximum of the response to a flash was more pronounced in a real cone than in the model.

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