The writer had shown in his early publications on light effects that aside from the heliotropic reaction of animals, which as we now know depends upon the product of the intensity and duration of illumination, there is a second reaction which depends upon the sudden changes in the intensity of illumination. These latter therefore obey a law of the form: Effect = f (di/dt).[227] Jennings has maintained that the heliotropic reactions of unicellular organisms are all of this kind, but investigations by Torrey and by Bancroft[228] on _Euglena_ have shown that Jennings’s statements were based on incomplete observations. [227] Loeb, J., _Arch. f. d. ges. Physiol._, 1893, liv., 81; _Jour. Exper. Zoöl._, 1907, iv., 151. [228] Bancroft, F. W., _Jour. Exper. Zoöl._, 1913, xv., 383. 4. In these experiments only one source of light was applied. “When two sources of light of equal intensity and distance act simultaneously upon a heliotropic animal, the latter puts its median plane at right angles to the line connecting the two sources of light.”[229] This fact has been amply verified by Bohn, by Parker and his pupils, and especially by Bradley Patten, who used it to compare the relative efficiency of two different lights. [229] Loeb, J., _Studies in General Physiology_, Chicago, 1905, p. 2. The behaviour of the animals under the influence of two lights is a confirmation of our theory of heliotropism inasmuch as the animal moves in such a direction that the symmetrical elements of the surface of the body are struck by light of the same intensity at the same angle, so that as a consequence equal masses of photosensitive substances are produced in symmetrical elements of their eyes or skin in equal times. The effect on the symmetrical muscles will be identical. As soon as one of the lights is a little stronger the animal will deviate towards this light, in case it is positively heliotropic and towards the weaker light if it is negatively heliotropic. This deviation again is not the product of chance but follows a definite law as Patten[230] has recently shown. He used the negatively heliotropic larvæ of the blowfly. These larvæ were made to record their trail while moving under the influence of the two lights. The results of the measurements of 2500 trails showing the progressive increase in angular deviation of the larvæ (from the perpendicular upon the line connecting the two lights), with increasing differences between the lights, are given in the following table. Since the deviation or angular deflection of the larvæ is towards the weaker of the two lights it is marked negative. [230] Patten, Bradley M., _Am. Jour. Physiol._, 1915, xxxviii., 313. TABLE VIII