MODE IN WHICH LIGHT ACTS ON THE RETINA. 273 



XIIL ON THE MODE IN WHICH LIGHT ACTS ON 

 THE ULTIMATE NERVOUS STRUCTURES OF 

 THE EYE, AND ON THE RELATIONS BETWEEN 

 SIMPLE AND COMPOUND EYES.* 



SINCE the publication, in 1826, of Joh. Miiller's Vergleichende 

 Physiologie des G-esichtssinnes, physiologists have admitted three 

 fundamental forms of the organ of vision. 1st, The eye-spot, 

 organised 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 continu- 

 ous. The difference between the simple and compound eye, 

 as explained by Miiller, 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 lenticular structures 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 of 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 nervous 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 



* Read before the Royal Society of Edinburgh, April 6, 1857. 

 T 



