PROPERTIES OF THE RETINA. 317 



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. 136. 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 y%V 



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. 

 The end reaction of the activity of a sensory nerve is a state of con- 

 sciousness. The variations in magnitude of this state can not be 

 measured with objective exactness, they must be judged subjectively 

 by the individual concerned. A stimulus too weak to give a re- 

 sponse with a motor nerve is usually designated in physiology as 

 subminimal ; a similar stimulus with sensory nerves is frequently 

 expressed by the equivalent term subliminal, that is, below the 

 threshold. So a stimulus just strong enough to provoke a percep- 

 tible reaction is the minimal stimulus for efferent nerves and the 

 threshold stimulus for sensory nerves. Inasmuch as the variations 

 in the intensity of consciousness can not be adequately measured, 

 it is customary, in studying the relations of the strength of stimulus 

 to the conscious response, to pay attention to the strength of stimu- 

 lus under any given condition which is sufficient to arouse a just 

 perceptible difference in the conscious reaction. Proceeding upon 

 this method, it is found in the case of the visual sensations and the 

 optic nerve, as with other sensations and their corresponding nerves, 

 that the increase of stimulus necessary to cause a just perceptible 

 change in consciousness varies with the amount of stimulus already 



