THE SENSE OF SIGHT. 



657 



tance of 5 meters, would have, therefore, a vertical and a horizontal dimen- 

 sion of 5 times 1.454 mm. or 7.27 mm. (Fig. 317 A), and at 10 meters corre- 

 sponding dimension of 14.54 mm., etc. (Fig. 317 B.) 



If with the accommodation suspended, the emmetropic eye could 

 clearly distinguish at a distance of 5 meters a letter 7.27 mm. in size which 

 would, therefore, subtend an angle of 5 minutes, then the acuity of the 

 vision would be normal and could be expressed as follows: V=|- or V = T. 



E T 



A. 



EL 



FIG. 317. STANDARD TEST LETTERS, FOR TESTING THE ACUITY OF VISION. 



If on the contrary at this distance the smallest letter that could be clearly 

 seen is one that would subtend an angle of 5 minutes at a distance of 10 meters 

 then the visual acuity would be only one-half the normal and could be 

 expressed as follows V =-f$- orV=J, etc. The acuity of vision is expressed, 

 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 

 42 



* 



FIG. 318. THE REFRACTION OF PARALLEL AND 

 DIVERGENT RAYS IN THE EMMETROPIC EYE IN THE 

 PASSIVE AND IN THE ACTIVE OR ACCOMMODATED 

 CONDITION. 



