VISION. 



801 



CIRCLES OF DIFFUSION. When rays of light proceeding from a 

 luminous object do not come to a focus directly on the retina, the 

 image is no longer distinct, and circles of diffusion appear about it. 

 In the normal condition, the luminous rays passing to a point of the 

 retina through the pupil form a cone, the base of which is at the 

 pupil and the apex at the retinal focus. But if the focus be placed 



Fig. 350. Schemer's Experiment an experiment to determine the 

 minimum distance of distinct vision. 



in front of or behind the retina, the latter intersects the bundle of 

 rays so that, instead of a point on the retina corresponding to one 

 on the luminous object, we have a circle formed. The different 

 points of the retina will be intersected by rays coming from various 

 parts of the object, and the image in this way becomes blurred and 



B C 



Fig. 351. Diagram to show that the visual angle and size of the retinal 

 image vary with the distance of the object from the eye. (BALL.) 



The image of S-B is seen at O.S. under the angle g, and the image of 

 T.C. is seen at I-W under the angle Ma. 



loses its distinctness. The existence of these circles of diffusion 

 explains why it is that we cannot at the same time see clearly objects 

 which are placed at different distances from the eye. Their size 

 varies with the distance of the focus from the retina, being larger 

 as the distance is greater, and also with the size of the pupil, con- 



