PHYSIOLOGY OF THE EYE. 



The eye is the peripheral organ of vision. By means of its 

 peculiar physical structure rays of light from external objects are 

 focused upon the retina and there set up nerve impulses that are 

 transmitted by the fibers of the optic nerve and optic tract to the 

 visual center in the cortex of the brain, where they arouse that 

 reaction in consciousness which we designate as a visual sensation. 

 In studying the physiology of vision we may consider the eye first 

 as an optical instrument physically adapted to form an image on 

 the retina and provided with certain physiological mechanisms for its 

 regulation ; and secondly we may study the properties of the retina 

 in relation to its reactions to light and the visual sensations them- 

 selves, or the physiology of the visual center in the brain. 



CHAPTER XVII. 



THE EYE AS AN OPTICAL INSTRUMENT DIOPTRICS 

 OF THE EYE. 



Formation of an Image by a Biconvex Lens. That the re- 

 fractive surfaces of the eye form an image of external objects upon 

 its retinal surface is a necessary conclusion from its physical struc- 

 ture. The fact may be demonstrated directly, however, by ob- 

 servation upon the excised eye of an albino rabbit. The thin coats 

 of such an eye are semitransparent, and if the eye is placed in a tube 

 of blackened paper and held in front of one's own eyes it can be seen 

 readily that a small, inverted image of external objects is formed 

 upon the retinal surface, just as an inverted image of the exterior is 

 formed upon the ground glass plate of a photographic camera. This 

 image is formed in the eye by virtue of the refractive surfaces of the 

 cornea and the lens. The curved surfaces of these transparent bodies 

 act substantially like a convex glass lens, and the physics of the 

 formation of an image by such a lens gives the simplest explanation 

 of the refractive processes in the eye. To understand the formation 

 of an image by a biconvex lens the following physical facts must be 



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