DIOPTRICS OF THE EYE. 313 



we have = -r-, or x = 0.341 mm., which is about the diam- 



eter of the fovea centralis. The retinal image of the object in 

 this case would be, in round numbers, about -nnrVinr f the actual 

 height of the object. 



Accommodation of the Eye for Objects at Different Dis- 

 tances. The normal or, as it is sometimes named, the emmetropic 

 eye, is arranged to focus parallel rays more or less accurately upon 

 the retina. That is, the refractive media have such curvatures and 

 densities that parallel, or substantially parallel rays are brought to 

 a focus upon the retinal surface. When objects are brought closer 

 to the eye, however, the rays proceeding from them become more 

 and more divergent. If the eye remains unchanged the refracted 

 rays cut the retina before' coming to a focus so that each 

 luminous point in the object, instead of forming a point upon the 



Fig. 129. Diagram explaining the change in the position of the image reflected from the 

 anterior surface of the crystalline lens. (Williams, after Donders.) 



retina, forms a circle, known as a diffusion circle. As this 

 is true for each point of the object, the retinal image as a whole 

 is blurred. We know, however, that up to a certain point 

 at least this blurring does not occur when the object is brought 

 closer to the eyes. The eye, in fact, accommodates itself to the 

 nearer object so as to obtain a clear focus. In a photographic 

 camera this accommodation or focusing is effected by moving the 

 ground glass plate farther away as the object is brought closer to the 

 lens. In the eye the same result is obtained by increasing the curva- 

 ture and therefore the refractive power of the lens. That a change 

 in the lens is the essential factor in accommodation for near objects 

 is demonstrated by a simple and conclusive experiment devised by 



