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:-- [113] Exner has since determined by measurement and calculation the optical properties of the eye of Hydrophilus. He finds that the focus of a corneal lens is about 3mm. away, and altogether behind the eye. [114] Zur vergl. Phys. des Gesichtsinnes. [115] A critical history of the whole discussion is to be found in Grenacher’s “Sehorgan der Arthropoden” (1879), from which we take many historical and structural details. 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.