VI 



DIOPTRIC MECHANISM OF THE EYE 



277 



Embryology and a recent Monograph l>y (Jirincione). The vitreous 

 body, the uvea (choroid, ciliary /one, iris), and the outer coat 

 (sclerotic and cornea) arc niesotlrnn;il in origin; the inner coat 

 alone is ectodermal in origin, and of the parts which constitute 

 it (retina, zonule, crystalline lens) only the inner layer of the 

 retina had in the adult the ch.iracter of a sensorial tissue, 

 composed, as we shall see, of spccilic, neural elements the outer 

 layer remains as a single stratum of pigmented epithelial cells, in 

 close functional relation with the nerve-cells of the retina. 



Giambattista della Porta (1589) first discovered the optical 

 instrument known as the camera obscura, which he compared to 

 the eye, noting the correspondence of its parts : the convergent lens 



Kir;. 110. Choroid membrane ami his exposed l>\ removal of Hie .sclerotic and cornea. Twice the 

 natural si/e. (/inn.) , pail of I he selemlir turned back; b, ciliaiy muscle; ., iris; e, 

 one of the ciliary nerves ; /, one of the vasa vorticosa or choroidal veins. 







of the camera obscura corresponds to the crystalline lens of the 

 eye; the diaphragm corresponds to the iris, while the surface on 

 which the reduced and inverted image of external objects is formed 

 in the camera corresponds to the retinal surface of the eye. The 

 dioptrics of the eye were first worked out systematically by 

 Kepler (1602). But neither della Porta nor Kepler saw the 

 formation of images on the retina. This was first demonstrated 

 by Christoph Schemer, a Jesuit Father, who observed it on the 

 excised eye of freshly killed animals (1609), and also on the 

 human eye when the retina was exposed at the back of the globe 

 (1625). 



The simplest method of seeing the formation of images at the 

 back of the eye is that of Magendie (1S'56). He dissected the eye 

 of an albino rabbit, and then, after removing all portions of the 



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