82 STUDIES IN GENERAL PHYSIOLOGY 
1. The direction of the median plane, or the direction of 
the progressive movements of an animal, coincides with the 
direction of the rays of light.’ 
2. The more refrangible rays of the visible spectrum are 
exclusively or more effective, than the less refrangible 
rays, in causing the orientation of the animals, as is also 
the case in plants. 
3. Light of a constant intensity acts as a constant stimu- 
lus in animals as well as in plants. 
4, The intensity of the light is of importance in animal 
heliotropism, in so far as only light of a certain intensity can 
cause heliotropic movements, and in so far as with an increase 
in the intensity the orientation of the animals toward the 
source of light becomes more exact. Direct sunlight causes 
winged insects (ants, Lepidoptera, plant lice, etc.) to fly, 
while diffuse light usually causes them only to creep. Posi- 
tively heliotropic animals will move toward the source of 
light, even if in so doing they go from places of greater 
intensity of light to places of less intensity ; negatively helio- 
tropic animals move away from the source of light, even if 
in so doing they pass from regions of less intense light to 
regions of greater intensity. 
5. Heliotropic movements occur only between certain 
limits of temperature. An optimum temperature lies beween 
these two limits at which the heliotropic movements occur 
most rapidly and precisely. This holds true also in plants. 
II. The orientation of an animal toward a source of light 
depends on the form of the body, just as the orientation of a 
plant to light depends on the form of the plant. 
1, Symmetrical points on the surface of the body of dor- 
siventral animals possess equal irritabilities. ? 
1If there is only a single source of light. If there are two sources of light of 
different intensities, the animal is oriented by the stronger of the two lights. If their 
intensities be equal, the animal is oriented in such a way as to have symmetrical 
points of its body struck by the rays at the same angle. [1903] 
2Equal in magnitude, not in direction. [1903] 
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