Müller’s theory is supported by Boll, Grenacher, Lubbock, Watase, and especially by Exner, who has given much attention to the subject of the vision of insects, and is the weightiest authority on the subject.
Gottsche’s view that each of the facetted eyes makes a distinct image which partially overlaps and is combined with all the images made by the other facets, was shown by Grenacher to be untenable, after repeating Gottsche’s experiments with the eyes of moths, in which the crystalline cones are firm and attached to the cornea. He was thus able to remove the soft parts, and to look through the cones and the cornea. When the microscope was focussed at the inner end of the cone, a spot of light was visible, but no image. As the object-glass was moved forward, the image gradually came into view, and then disappeared again. Here, then, the image is formed in the interior of the cone itself.
Exner attempted to make this experiment with the eye of Hydrophilus, but in that insect the crystalline cones always came away from the cornea. “He, however, calculated the focal length, refraction, etc., of the cornea, and concluded that, even if, in spite of the crystalline cone, an image could be formed, it would fall much behind the retinula.”
“In these cases, then,” adds Lubbock, “an image is out of the question. Moreover, as the cone tapers to a point, there would, in fact, be no room for an image, which must be received on an appropriate surface. In many insect eyes, indeed, as in those of the cockchafer, the crystalline cone is drawn out into a thread, which expands again before reaching the retinula. Such an arrangement seems fatal to any idea of an image.”
Lubbock thus sums up the reasons which seem to favor Müller’s theory of mosaic vision, and to oppose Gottsche’s view: “(1) In certain cases, as in Hyperia, there are no lenses, and consequently there can be no image; (2) the image would generally be destroyed by the crystalline cone; (3) in some cases it would seem that the image would be formed completely behind the eye, while in others, again, it would be too near the cornea; (4) a pointed retina seems incompatible with a clear image; (5) any true projection of an image would in certain species be precluded by the presence of impenetrable pigment, which only leaves a minute central passage for the light-rays; (6) even the clearest image would be useless, from the absence of a suitable receptive surface, since both the small number and mode of combination of the elements composing that surface seem to preclude it from receiving more than a single impression; (7) no system of accommodation has yet been discovered; finally (8), a combination of many thousand relatively complete eyes seems quite useless and incomprehensible.”
In his most recent work (1890) on the eyes of crustacea and insects, Exner states that the numerous simple eyes which make up the compound eye have each a cornea, but it is more or less flat, and the crystalline part of the eye has not the shape of a lens, but of a “lens cylinder,” that is, of a cylinder which is composed of sheets of transparent tissue, the refracting powers of which decrease toward the periphery of the cylinder. If an eye of this kind is removed and freed of the pigment which surrounds it, objects may be looked at through it from behind; but its field of vision is very small, and the direct images received from each separate eye are either produced close to one another on the retina (or rather the retinulæ of all the eyes) or superposed. In this last case no less than thirty separate images may be superposed, which is supposed to be of great use to night-flying insects. Exner claims that many other advantages result from the compound nature of an insect’s eye. Thus the mobile pigment, which corresponds to our iris, can take different positions, either between the separate eyes or behind the lens cylinders, in which case it acts as so many screens to intercept the over-abundance of light. Exner finds that with its compound eyes the common glow-worm (Lampyris) is capable of distinguishing large signboard letters at a distance of ten or more feet, as well as extremely fine lines engraved one-hundredth of an inch apart, if they are at a distance of less than half an inch from the eye. Exner substantiates the truth of the results of Plateau’s experiments, and claims that while the compound eye is inferior to the vertebrate eye for making out the forms of objects, it is superior to the latter in distinguishing the smallest movements of 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.