PROPERTIES OF THE RETINA. 341 



tion. The estimates given are for ordinary room light. Out-of- 

 doors, and especially in the case of persons who live habitually 

 an outdoor life, visual acuity or the power of visual discrimination 

 is increased. We may believe that under the most favorable con- 

 ditions of illumination and contrast we can resolve two objects 

 whose images on the fovea are separated by a distance about equal 

 to the diameter (0.002 mm.) of a single cono. The acuity of vision 

 does not varj^ greatly throughout the fovea; any object whose 

 retinal image falls well within the fovea can be seen quite dis- 

 tinctly in all of its parts when the eye is fixed for the center of the 

 object. This is the case, for instance, with the moon. Neverthe- 

 less, in looking at such an object as the moon the eye, to make out 

 details, will fixate one point after another, showing that for most 

 distinct vision we use probably only the center of the fovea. As 

 we pass out from the fovea into the peripheral field of vision the 

 acuity of vision diminishes very rapidly, so that at 20 degrees, for 

 instance, from the center of the fovea the retinal images must be 

 separated by a distance of 0.035 mm. in order to be recognized 

 as distinct; a distance ten times as great as is necessary in the 

 Fovea. On this account our vision in the peripheral field is very 

 indistinct, — details of form cannot be clearly perceived. The 

 rapidity with which visual acuity diminishes as we pass outward 

 from the fovea is indicated by the curve given in Fig. 145. In all 

 close work, therefore, we keep our eyes moving continually so as to 

 bring one point after another into the ce^lter of the fovea, as is well 

 illustrated by the act of reading. If the eye is kept fixed upon the 

 central letter of a long word, only one or two letters on each side 

 can be made out distinctly in spite of the fact that with such 

 famihar objects we can guess the letter even when the image is not 

 entirely distinct. In ophthalmological practice the acuity of vision 

 (central vision) is measured usually by test letters whose size is 

 such that at the distance at which they are read — say, 6 meters (20 

 feet), the practical far point at which no accommodation is needed — 

 the height of the letter subtends an angle of 5', while the width of 

 the fines composing the letters subtends an angle of 1'. An eye 

 that can distinguish the letters at this distance is said to be nor- 

 mal; one that can distinguish them only at a smaller distance or 

 at the given distance requires letters of larger size has a sub- 

 normal acuity of vision. If, for instance, an individual at 20 feet 

 can read only those letters that the normal eye can distinguish 

 at 100 feet his visual acuity, V, is equal to 3^. The general ex- 

 pression used is V = ^, in which V indicates the visual acuity; 

 d, the distance (20 feet) at which the test is made, and D, the 

 distance at which the normal eye could read the smallest letters 

 recognized by the person examined at distance d. 



