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4. On the mode in which Light acts on the Ultimate Nervous 

 Structures of the Eye, and on the relations between Simple 

 and Compound Eyes. By Professor Goodsir. 



Since the publication in 1826, of Joh. Miiller's Vergleichende Phy- 

 siologie des Geskhtssinnes, Physiologists have admitted three fun- 

 damental forms of the organ of vision. \st. 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. Zd, The simple eye, 

 ia which the retinal picture is continuous. The difference between 

 the simple and compound eye, as explained by MuUer, and since 

 generally admitted, consists in this, that the formation of the pic- 

 ture in the simple eye is the result of the convergence of all the 

 pencils diverging from the visible points of the object on correspond- 

 ing points of the retina, by means of the crystalline lenticular struc- 

 ture of the organ ; while, in the compound eye, the picture is 

 formed by the stopping off, by means of the constituent crystalline 

 columns ofj the eye, all rays except those which pass in or near the 

 axes of the columns. The extent of surface of any object, and the 

 number of separate parts of such surface, represented on the ner- 

 vous structure of a compound eye, will vary, therefore, in terms of 

 the distance of the object, the curvature of the superficial ocular 

 surface, the corresponding inclination of the crystalline columns to 

 one another, the size of their individual transverse sections, and 

 their lengths. The continuous retinal picture in the simple eye is 

 psychically interpreted as a continuous image. If, therefore, the 

 possessor of a compound eye perceives a continuous imatre of an ob- 

 ject, it must be the result of a more complex psychical operation, in 

 virtue of which, the separate portions of the ocular mosaic picture 

 are psychically combined, and interpreted as a continuous whole. 



The successive researches of Treviranus, Gottsche, Hannover, 

 Pacini, H. Muller, and KoUiker, have determined the existence and 

 general structure of close-set rods or columns, which extend be- 

 tween the inner and outer surfaces of the retina, in the midst of the 

 nervous and vascular textures of that membrane. The outer ex- 

 tremities of these rods present a crystalline columnar aspect, and 

 constitute, collectively, the external layer of the retina, usually 



