Let us assume that the negatively heliotropic animal is at an equal distance from the two unequal lights and placed so that at the beginning of the experiment its median plane is at right angles to the line connecting the two lights, but with its head turned away from them. In that case the velocity of reaction in the symmetrical photosensitive elements of the eyeless larvæ is greater on the side of the stronger light. Since the animal is negatively heliotropic this will result in a greater relaxation or a diminution of the energy production of the muscles turning the head of the animal towards the side of the stronger light. Hence the animal will automatically deviate from the straight line towards the side of the weaker light. By the alteration of the position of its body the photosensitive elements exposed to the stronger of the two lights will be put at a less efficient angle and hence the rate of photochemical reaction on this side will be diminished. The deviation from the perpendicular in which the animal will ultimately move will be such that as a consequence, the rate of photochemical reaction in symmetrical elements is again equal. The ultimate direction of motion will, according to our theory always be such that the mass of chemical products formed under the influence of light in symmetrical photosensitive elements during the same time is equal.
Patten also investigated the question whether the same difference of percentage between two lights would give the same deviation, regardless of the absolute intensities of the lights used. The absolute intensity was varied by using in turn from one to five glowers. The relative intensity between the two lights varied in succession by 0, 8-1/3, 16-2/3, 25, 33-1/3, and 50 per cent. Yet the angular deflections were within the limits of error identical for each relative difference of intensity of the two lights no matter whether, 1, 2, 3, 4, or 5 glowers were used. The following table shows the result.
A TABLE BASED ON THE MEASUREMENTS OF 2700 TRAILS SHOWING THE ANGULAR DEFLECTIONS AT FIVE DIFFERENT ABSOLUTE INTENSITIES
---------+----------------------------------------------------------- | _Difference of Intensity between the Two Lights_ _Number +---------+---------+---------+---------+---------+--------- of | 0 | 8-1/3 | 16-2/3 | 25 | 33-1/3 | 50 Glowers_|per cent.|per cent.|per cent.|per cent.|per cent.|per cent. ---------+---------+---------+---------+---------+---------+--------- | _Deflection in Degrees_ 1 | -0.55 | -2.32 | -5.27 | -9.04 | -11.86 | -19.46 2 | -0.10 | -3.05 | -6.12 | -8.55 | -11.92 | -22.28 3 | +0.45 | -2.60 | -5.65 | -8.73 | -13.15 | -20.52 4 | -0.025 | -2.98 | -6.60 | -9.66 | -11.76 | -19.88 5 | -0.225 | -2.92 | -5.125 | -8.30 | -10.92 | -19.28 ---------+---------+---------+---------+---------+---------+--------- Average | -0.09 | -2.77 | -5.75 | -8.86 | -11.92 | -20.28 ---------+---------+---------+---------+---------+---------+---------