PHYSIOLOGY OF THE EYE. 



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

 physical structure rays of light from external objects are focused 

 upon the retina and there set up nerve impulses that are trans- 

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

 visual center in the cortex of the brain. In this last organ is 

 aroused 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 physi- 

 ological mechanisms for its regulation; secondly, we may study 

 the properties of the retina in relation to its reactions to light,, 

 and lastly, the visual sensations themselves, or the physiology 

 of the visual center in the lirain. 



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 

 the retinal surface is a necessary conclusion from the 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 may be used to explain the 

 refractive processes in the eye. To understand the formation 

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

 borne in mind. Parallel rays of light falling upon one surface of the 

 lens are brought to a point or focus (F) behind the other surface 

 (Fig. 124). This focus for parallel rays is the principal focus and 

 the distance of this point from the lens is the p-incipal focal dis- 

 tance. This distance depends upon the curvature of the lens and 



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