Heliotropic Animals as Photometers on the Basis of the Validity of the Bunsen-Roscoe Law for Heliotropic Reactions.

While it was customary to express animal instincts in terms of human behavior, one of us many years ago began to replace this anthropomorphic. method by the objective and quantitative methods of the physicist. These methods were most easily applicable in the case of those instincts familiar to every layman in which animals were apparently attracted or repelled by light. Loeb1 was able to express the effect of light in these cases in the following terms: Certain animals are compelled automatically to orient their body in such a way that symmetrical elements of their photosensitive surface are struck by light of the same intensity. In that case the tension and energy production in the symmetrical muscles of both sides of the body are equal and there is no reason for the animal to deviate from this direction of its motion. If, however, the symmetrical photosensitive elements (e.g., the eyes) receive unequal illumination the tension or energy production of the symmetrical muscles is no longer the same and the animal is automatically turned until its orientation is again such that symmetrical photosensitive elements receive the same amount of light. It was obvious from the observations that this reaction was a function of the constant intensity of light and Loeb assumed that it was a photochemical effect and that the function was probably the law of Bunsen and Roscoe, whereby the effect equals the product i t, where i is the intensity of light and t the duration of illumination.2