THE EYE AS AN OPTICAL INSTRUMENT. 



505 



the red light emanating from the spots; consequently only 

 some of its end-organs and nerve-fibres would be stimulated 

 and the result would be the recognition of two separate red 



FIG. 189. Diagram illustrating the indistinctness of vision with a retina alone. 

 K, a surface on which are two spots, A and B; r i\ the retina. The diverging 

 lines represent rays of light spread uniformly over the retina from each spot. 



objects. In our eyes there are certain refracting media 

 which lie in front of the retina and take the place of the lens 

 L in Fig. 140. That portion of physiology which treats of 



FIG. 140. Illustrating the use of a lens in giving definite retinal images. A, B, 

 K, r r, as in Fig. 139. Z,, a biconvex lens so placed that it l>rings to a focus on the 

 points a and b. of the retina, rays of light diverging from ,-1 and B respectively. 



the physical action of these media or, in other words, of the 

 eye as an optical instrument, is known as the dioptrics of the 

 eye. 



The Appendages of the Eye. The eyeball itself con- 

 sists of the retina and refracting media, together with sup- 

 porting and nutritive structures and other accessory appa- 

 ratuses, as, for example, some controlling the light-converg- 

 ing power of the media, and others ^regulating the size of the 

 aperture (pupil] by which light enters. Outside the ball lie 

 muscles which bring about its movements, and other parts 

 serving to protect it. 



Each orbit is a pyramidal cavity occupied by connective 

 tissue, muscles, blood-vessels and nerves, and in great part by 

 fat, which forms a soft cushion on which the back of the eye- 

 ball lies and rolls during its movements. The contents of 



