Comments on the testing of two prominent dark-adaptation hypotheses.

There appear to be some logical errors in recent attempts to test the hypotheses. (I) that the process of dark adaptation is controlled by the concentration of a substance in the receptors (the photochemical hypothesis) and (2) that the state of the visual system during dark adaptation is equivalent to that produced by some stabilized steady background (the equivalent-background hypothesis). The various factors that should be considered in testing these hypotheses are discussed.