Since the publication in 1826, of Joh. Müller's Vergleichende Physiologie des Gesichtssinnes, Physiologists have admitted three fundamental forms of the organ of vision. 1st, The eye-spot, organized for the mere perception of light. 2d, The compound eye, in which the picture on the nervous surface is a mosaic. 3d, The simple eye, in which the retinal picture is continuous. The difference between the simple and compound eye, as explained by Mülier, and since generally admitted, consists in this, that the formation of the picture in the simple eye is the result of the convergence of all the pencils diverging from the visible points of the object on corresponding points of the retina, by means of the crystalline lenticular structure of the organ; while, in the compound eye, the picture is formed by the stopping off, by means of the constituent crystalline columns of the eye, all rays except those which pass in or near the axes of the columns.