THE SENSE OF SIGHT 



709 



FIG. 298. THE REFRACTION OF PARALLEL AND 

 DIVERGENT RAYS IN THE EMMETROPIC EYE IN THE 

 PASSIVE AND IN THE ACTIVE OR ACCOMMODATED 

 CONDITION. 



therefore, by a fraction the numerator of which is the distance at which the 

 test is made and whose denominator is the distance at which the smallest 

 letters distinguished by the patient subtend an angle of 5 minutes, or in 

 other words the distance at 

 which the patient reads di- 

 vided by the distance at which 

 he ought to read the smallest 

 letters seen by him on the 

 chart. 



Accommodation. Ac- 

 commodation may be denned 

 as the power which the eye 

 possesses of adjusting itself to 

 vision at different distances; 

 or in other words, the power 

 of focusing rays of light on the 

 retina, which come from 

 different distances at different 

 times. That such a power 

 is a necessity is apparent from 

 the fact that it cannot focus 

 rays coming from a distant 

 and a near object at the same time. Thus, if an object is held before 

 one eye at a distance of 22 centimeters, for example, and the vision 

 is directed to a distant object it is evident that the near object is indistinctly 

 seen; but if the vision is then directed to the near object, it in turn becomes 

 clear and distinct, while the distant object becomes blurred and indistinct. 

 It is evident, therefore, that rays of light coming from a distant and a near 

 object cannot be simultaneously, but only alternately, focused on the retina. 

 The observer at the same time becomes conscious, as the vision is directed 

 from the distant to the near object, of a change in the eye itself, a change 

 that involves time and effort. The reasons for these facts will become 

 apparent from a consideration of the following facts: 



In a normal or emmetropic eye, parallel rays of light (Fig. 298, a, b) 

 after passing through the optic media are converged and brought to a 

 focus on the retina, /. Rays, however, which come from a luminous 

 point situated near the eye, P, and are therefore divergent, passing through 

 the optic media at the same time, are intercepted by the retina before they 

 are focused, and give rise to the formation of diffusion-circles and indis- 

 tinctness of vision. The reverse is also true. When the eye is adjusted 

 for the refraction and focusing of divergent rays (Fig. 298, P) parallel rays 

 will be brought to a focus before reaching the retina, and, again diverging, 

 will form diffusion-circles. It is evident, therefore, that it is impossible 

 to focus simultaneously both parallel and divergent rays, and to see dis- 

 tinctly at the same time, two objects which are situated at different distances. 

 The eye must be alternately adjusted first to one object and then to another. 

 To this adjustment the term accommodation has been given. 



The following table of Listing shows the size of the diffusion-circles 

 formed of objects situated at different distances when the accommodative 

 power is suspended in an emmetropic eye: 



