| Percentage Difference in the Intensity of the Two Lights | Average Angular Deflection of the Two Paths of the Larvæ towards the Weaker Light | |
|---|---|---|
| Per Cent. | Degrees | |
| 0 | 0-0.09 | |
| 8 | 1⁄3 | 0-2.77 |
| 16 | 2⁄3 | 0-5.75 |
| 25 | 0-8.86 | |
| 33 | 1⁄3 | -11.92 |
| 50 | -20.28 | |
| 66 | 2⁄3 | -30.90 |
| 83 | 1⁄3 | -46.81 |
| 100 | -77.56 | |
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, 81⁄3, 162⁄3, 25, 331⁄3, 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.
TABLE IX
A Table Based on the Measurements of 2700 Trails Showing the Angular Deflections at Five Different Absolute Intensities
| Number of Glowers | Difference of Intensity between the Two Lights | |||||
|---|---|---|---|---|---|---|
| 0 per cent. | 81⁄3 per cent. | 162⁄3 per cent. | 25 per cent. | 331⁄3 per cent. | 50 per cent. | |
| Deflection in Degrees | ||||||
| 1 | -0.550 | -2.32 | -5.270 | -9.04 | -11.86 | -19.46 |
| 2 | -0.100 | -3.05 | -6.120 | -8.55 | -11.92 | -22.28 |
| 3 | +0.450 | -2.60 | -5.650 | -8.73 | -13.15 | -20.52 |
| 4 | -0.025 | -2.98 | -6.600 | -9.66 | -11.76 | -19.88 |
| 5 | -0.225 | -2.92 | -5.125 | -8.30 | -10.92 | -19.28 |
| Average | -0.090 | -2.77 | -5.750 | -8.86 | -11.92 | -20.28 |
Such constancy of quantitative results is only possible where we are dealing with purely physicochemical phenomena or where life phenomena are unequivocally determined by purely physicochemical conditions.
5. It seems difficult for some biologists, even with the validity of the Bunsen-Roscoe law proven, to imagine that the movements of the animals under the influence of light are not voluntary (or not dictated by the mysterious “trial and error” method of Jennings).[231] But one wonders how it is possible on such an assumption to account for the fact that the angle of deflection of the larva of the fly when under the influence of two lights of different intensities should be always the same for a given difference in intensity; or why the time for curvature in Eudendrium should vary inversely with the intensity of illumination. It is, however, possible to complete the case for the purely physicochemical analysis of these instincts. John Hays Hammond, Jr., has succeeded in constructing heliotropic machines which in the dark follow a lantern very much in the manner of a positively heliotropic animal. The eyes of this heliotropic machine consist of two lenses in whose focus is situated the “retina” consisting of selenium wire. The two eyes are separated from each other by a projecting piece of wood which represents the nose and allows one eye to receive light while the other is shaded. The galvanic resistance of selenium is altered by light; and when one selenium wire is shaded while the other is illuminated, the electric energy (supplied by batteries inside the machine) which makes the wheels turn (these take the place of the legs of the normal animal) no longer flows symmetrically to the steering wheel, and the machine turns towards the light. In this way the machine follows a lantern in a dark room in a way similar to that of a positively heliotropic animal. Here we have a model of the heliotropic animal whose purely mechanistic character is beyond suspicion, and we may be sure that it is not “fondness” for light or for brightness nor will-power nor a method of “trial and error” which makes the machine follow the light.
