objects in the total field of vision. More recently Mallock has given some optical reasons to show that Müller’s view is the true one. He concludes, and thus agrees with Plateau, that insects do not see well, at any rate as regards their power of defining distant objects, and their behavior certainly favors this view. It might be asked, What advantage, then, have insects with compound eyes over those with simple eyes? Mallock answers, that the advantage over simple-eyed animals lies in the fact that there is hardly any practical limit to the nearness of the objects they can examine. “With the composite eye, indeed, the closer the object the better the sight, for the greater will be the number of lenses employed to produce the impression; whereas, in the simple eye the focal length of the lens limits the distance at which a distinct view can be obtained.” He gives a table containing measures of the diameters and angles between the axes of the lenses of various insect eyes, and states that the best of the eyes would give a picture about as good as if executed in rather coarse woodwork and viewed at a distance of a foot, “and although a distant landscape could only be indifferently represented on such a coarse-grained structure, it would do very well for things near enough to occupy a considerable part of the field of view.” =The principal use of the facetted eye to perceive the movements of animals.=—Plateau adopts Exner’s views as to the use of the facetted eye in perceiving the movements of other animals. He therefore concludes that insects and other arthropods with compound eyes do not distinguish the form of objects; but with Exner he believes that their vision consists mainly in the perception of moving bodies. Most animals seem but little impressed by the form of their enemies or of their victims, though their attention is immediately excited by the slightest displacement. Hunters, fishermen, and entomologists have made in confirmation of this view numerous and demonstrative observations. Though the production of an image in the facetted eye of the insect seems impossible, we can easily conceive, says Plateau, how it can ascertain the existence of a movement. Indeed, if a luminous object is placed before a compound eye, it will illuminate a whole group of simple eyes or facets; moreover, the centre of this group will be clearer than the rest. Every movement of the luminous body will displace the centre of clearness; some of the facets not illuminated will first receive the light, and others will reënter into the shade; some nervous terminations will be excited anew, while those which were so formerly will cease to be. Hence the facetted eyes are not complete visual organs, but mainly organs of orientation.