In vision we are dealing with a certain definite stimulus--light, with its two modifications, colour and motion. Insects have two sets of organs for vision, the faceted eye and the so-called simple eye, or ocellus. These have been historically derived from one and the same organ. In order to exercise the function of sight the facets need a greater pencil of light rays by night than by day. To obtain the same result we dilate the pupil. But nocturnal insects are dazzled by the light of day, and diurnal insects cannot see by night, for neither possess the faculty of accommodation. Insects are specially able to perceive motion, but there are only very few insects that can see distinctly. For example, I watched one day a wasp chasing a fly on the wall of a veranda, as is the habit of this insect at the end of summer and in the autumn. She dashed violently in flight at the flies sitting on the wall, which mostly escaped. She continued her pursuit with remarkable pertinacity, and succeeded on several occasions in catching a fly, which she killed, mutilated, and bore away to her nest. Each time she quickly returned to continue the hunt. In one spot of the wall was stuck a black nail, which was just the size of a fly, and I saw the wasp very frequently deceived by this nail, upon which she sprang, leaving it as soon as she perceived her error on touching it. Nevertheless, she made the same mistake with the nail shortly after. I have often made similar observations. We may certainly conclude that the wasp saw something of the size of a fly, but without distinguishing the details; therefore she saw it indistinctly. Evidently a wasp does not only perceive motion; she also distinguishes the size of objects. When I put dead flies on a table to be carried off by another wasp, she took them, one after another, as well as spiders and other insects of but little different size placed by their side. On the other hand, she took no notice of insects much larger or much smaller put among the flies. Most entomologists have observed with what ingenuity and sureness dragon-flies distinguish, follow, and catch the smallest insects on the wing. Of all insects, they have the best sight. Their enormous convex eyes have the greatest number of facets. Their number has been estimated at 12,000, and even at 17,000. Their aerial chases resemble those of the swallows. By trying to catch them at the edge of a large pond, one can easily convince oneself that the dragon-flies amuse themselves by making sport of the hunter; they will always allow one to approach just near enough to miss catching them. It can be seen to what degree they are able to measure the distance and reach of their enemy.