MODE IN WHICH LKJIIT ACTS ON THK RETINA. L'73 



XITL— ON THE MODE TN WIITCII LKIIIT ACTS ON 

 THE ULTLMATK NKUVOUS STRUCTURES OF 

 THE EYE, AND OX THE RELATIONS BET\\T.EN 

 SIMrLE AND COMPOUND EYES.* 



Since the publication, iii 1826, of Joli. Mliller's Verghichcmh 

 Physiologic dc^ Gesichtssiimes, physiologists have admitted three 

 fundamental forms of the organ of vision. Is^, Tlie eye-spot, 

 organised for the mere perception of liglit ; 2r/, The compound 

 eye, in which the picture on the nervous surface is a mosaic ; 

 3rf, The simple eye, in which the retinal picture is continu- 

 ous. The difference between the simple and compound eye. 

 as explained by ^M idler, 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 stnictures of the organ ; 

 while, in tlie compound eye, the picture is funned by the 

 stopping off, by means of the constituent cr}'stallino columns 

 of the eye of all rays except those which pass in or near tlie 

 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 var}-, tlierefore, 

 in terms of the distance of the object, the cursature of the 

 superficial ocular surfiice, the corresponding inclination of the 

 crystalline columns to one another, the size of their individual 



* Read before tlie Royal .Soriety of Edinburgh, April 6, 1857. 



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