24 JACQUES LOEB AND HARDOLPH WASTENEYS 



of the angle of incidence. The influence of the Hght upon the 

 tension and action of symmetrical muscles must in such a case 

 be identical. The light, if it remains constant, will therefore 

 not cause the animal to alter the direction of its motions. If, how- 

 ever, the light strikes symmetrical elements at a different angle 

 the velocity of photochemical reaction is not the same in the two 

 symmetrical elements and the symmetrical muscles on both 

 sides of the animal will receive unequa impulses (by reflex) 

 from this source. This will lead to an automatic turning of the 

 animal until its plane of symmetry again falls into the direction 

 of the rays of light. 



If these premises were true, it followed that the heliotropic 

 reactions of animals should obey the law of Bunsen and Roscoe 

 which says that (within certain limits) the photochemical elTect 

 of light is equal to the product of the intensity into the duration 

 of illumination; and one of us predicted that this law would 

 probably be found to hold for animal heliotropism.^ This 

 prediction proved true for the heliotropic curvatures of Euden- 

 drium, as the experiments of Loeb and Ewald^ showed. Ewald 

 could also show that Talbot's law holds for the orientation of 

 the eye of Daphnia by light, ^ and Talbot's law is an expression 

 of the fact that the physiological effect of light is equal to the 

 product of intensity and duration of illumination. 



The same aw holds for the heliotropic reactions of plants, as 

 Blaauw and Froschl had shown. '^^ It also holds for the human 

 eye.^ In all these cases it should be remembered that the law 

 of Bunsen and Roscoe is a threshold law, inasmuch as it holds 

 only within certain limits. 



With the reduction of both groups of heliotropic reactions, 

 those of animals as well as of plants, to the same law, namely, 

 that of Bunsen and Roscoe, it is idle to consider further the 

 idea that animals are led to the light because they are "fond" 



* Loeb, J. The mechanistic conception of life. Chicago, 1912. 

 - Zentralbl. f. Physiol., Bd. 27, p. 1165, 1914. 

 « Science, N. S., vol. 38, p. 236, 1913. 



"Blaauw, Rec. des Travaux Botaniques Noerlandais, Bd. 5, p. 209, 1909; 

 Froschl, Sitzungsb. d. Akad. in Wien, 1908. 



8 Charpentier, Arch. d'Ophthalmol., torn. 10, 1890. 



