PROPERTIES OF THE RETINA. 345 



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 cone. The acuity of vision 

 does not vary 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 center 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 

 familiar 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 

 each subtends at the eye an angle of 5 minutes. An eye that can 

 distinguish the letters at this distance is said to be normal; one that 

 can distinguish them only at a smaller distance or at the given 

 distance requires letters of larger size has a subnormal 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 -ffa. 



Relation between the Amount of Sensation and the Intensity 

 of the Stimulus Threshold Stimulus. With the sensory as with 

 the motor nerves we may distinguish between various degrees of sub- 

 maximal stimulation. The stronger the stimulus, the stronger the 

 reaction, that is, in the case of the optic nerve, the visual sensation. 



