Night-flying moths were placed in a box and exposed in a room to ordinary light. As twilight approached the moths became active and began to fly always toward the window side of the box. They were positively heliotropic to light of this intensity. If let out of the case, they flew toward the window, where they remained even during the whole of the next day, fully exposed to light. If the moth is disturbed in the daytime, so that it flies, it goes always toward the light, and never away from it. These facts show that the moth is always positively heliotropic, and also that the flight toward the lamp is a natural response, misapplied in this case. That the moths do not fly by day is due to another factor, namely, the alternation in the degree of their sensitiveness at different times. But this condition alone does not seem to account fully for all the facts. If the moths are given the alternative of flying toward the evening light, or toward the lamp, they always go toward the brighter light. Thus if, when they swarm at dusk, they are set free in the middle of the room, at the back of which a lamp is burning, the moths fly toward the window. If, however, they are set free within a metre of the lamp, they fly toward it. The explanation that Loeb offers of the habit of these moths to fly only in the evening is, that, although they are at all times positively heliotropic, they respond to light only in the evening. In other words, it is assumed that there is a periodic change in their sensitiveness to light, which corresponds with the change from day to night. Loeb says that, just as certain flowers open only at night, and others only during the day, so do moths become more responsive in the evening, and butterflies during the day. Both moths and butterflies are positively heliotropic, and the sensitiveness of moths to light may be even greater in the evening than is that of butterflies, for the light of the evening to which the moth reacts is less than the minimal to which the butterfly responds. Moths appear to pass into a sort of sleep during the day, while butterflies are quiescent only at night. The periodicity of the sleeping time continues, at least for several days, when the insects are kept in the dark. For instance, moths kept in the dark become restless as the evening approaches, as Réaumur observed long ago. It has been found in plants that this sort of periodicity may continue for several days, but gradually disappears if the plants are kept in the dark. By using artificial light, and exposing the plants to it during the night, and putting them in the dark during the day, a new periodicity, alternating with the former one, may be induced; and this will continue for some days if the plants are then kept continually in the dark.