Fig. 54.—Section through Simple Eye of Vespa. The references as above. Simplified from Grenacher.
Fig. 55.—Diagrammatic section of Compound Eye. The references as above.
All the parts between the crystalline cones and the true optic nerve are considered by Hickson to compose the retina of Insects, which, instead of ending at the fenestrated-membrane, as has often been assumed, includes the ganglia and decussating fibres of the optic tract. The layer of retinulæ and rhabdoms does not form the whole retina, but merely that part which, in the vertebrate eye, is known as the layer of rods and cones.
As to the way in which the compound eye renders distinct vision possible, there is still much difference of opinion. A short review of the discussion which has occupied some of the most eminent physiologists and histologists for many years past will introduce the reader to the principal facts which have to be reconciled.
The investigation, like so many other trains of biological inquiry, begins with Leeuwenhoeck (Ep. ad Soc. Reg. Angl. iii.), who ascertained that the cornea of a shardborne Beetle, placed in the field of a microscope, gives images of surrounding objects, and that these images are inverted. When the cornea is flattened out for microscopic examination, the images (e.g., of a window or candle-flame) are similar, and it has been too hastily assumed that a multitude of identical images are perceived by the Insect. The cornea of the living animal is, however, convex, and the images formed by different facets cannot be precisely identical. No combined or collective image is formed by the cornea. When the structure of the compound eye had been very inadequately studied, as was the case even in Cuvier’s time (Leçons d’Anat. Comp., xii., 14), it was natural to suppose that all the fibres internal to the cornea were sensory, that they formed a kind of retina upon which the images produced by the facets were received, and that these images were transmitted to the brain, to be united, either by optical or mental combination, into a single picture. Müller,[113] in 1826, pointed out that so simple an explanation was inadmissible. He granted that the simple eye, with its lens and concave retina, produces a single inverted image, which is able to affect the nerve-endings in the same manner as in Vertebrates. But the compound eye is not optically constructed so as to render possible the formation of continuous images. The refractive and elongate crystalline cones, with their pointed apices and densely pigmented sides, must destroy any images formed by the lenses of the cornea. Even if the dioptric arrangement permitted the formation of images, there is no screen to receive them.[114] Lastly, if this difficulty were removed, Müller thought it impossible for the nervous centres to combine a great number of inverted partial images. How then can Insects and Crustaceans see with their compound eyes? Müller answered that each facet transmits a small pencil of rays travelling in the direction of its axis, but intercepts all others. The refractive lens collects the rays, and the pigmented as well as refractive crystalline cone further concentrates the pencil, while it stops out all rays which diverge appreciably from the axis. Each element of the compound eye transmits a single impression of greater or less brightness, and the brain combines these impressions into some kind of picture, a picture like that which could be produced by stippling. It may be added that the movements of the insect’s head or body would render the distance and form of every object in view much readier of appreciation. No accommodation for distance would be necessary, and the absence of all means of accommodation ceases to be perplexing. Such is Müller’s theory of what he termed “mosaic vision.” Many important researches, some contradictory, some confirmatory of Müller’s doctrine,[115] have since been placed on record, with the general result that some modification of Müller’s theory tends to prevail. The most important of the new facts and considerations which demand attention are these:—
Reasons have been given for supposing that images are formed by the cornea and crystalline cones together. This was first pointed out by Gottsche (1852), who used the compound eyes of Flies for demonstration. Grenacher has since ascertained that the crystalline cones of Flies are so fluid that they can hardly be removed, and he believes that Gottsche’s images were formed by the corneal facets alone. He finds, however, that the experiment may be successfully performed with eyes not liable to this objection, e.g., the eyes of nocturnal Lepidoptera. A bit of a Moth’s eye is cut out, treated with nitric acid to remove the pigment, and placed on a glass slip in the field of the microscope. The crystalline cones, still attached to the cornea, are turned towards the observer, and one is selected whose axis coincides with that of the microscope. No image is visible when the tip of the cone is in focus, but as the cornea approaches the focus, a bristle, moved about between the mirror and the stage, becomes visible. This experiment is far from decisive. No image is formed where sensory elements are present to receive and transmit it. Moreover, the image is that of an object very near to the cornea, whereas all observations of living Insects show that the compound eye is used for far sight, and the simple eye for near sight. Lastly, the treatment with acid, though unavoidable, may conceivably affect the result. It is not certain that the cones really assist in the production of the image, which may be due to the corneal facets alone, though modified by the decolorised cones.