316 



THE SPECIAL SENSES. 



logical point is smaller less than half the size given above. We 

 may believe, therefore, that under the most favorable conditions 

 we can perceive an object whose image on the fovea is less than 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 distinctly in all 

 of its parts when the eye is fixed for the center of the object. This 



0* 30 40 30 HT4 m ' 1T"4r Hf ZO 9 J0 W 



<50* 



60 



Fig. 136. Curve to show the relative acuity of vision in the central and peripheral 

 fields and in the light-adapted and the dark-adapted eye. (Koester.) The full line repre- 

 sents the relative acuteness of vision in the eye exposed to usual illumination. From the 

 center of the fovea, 0, the acuity of vision falls rapidly at first and then more slowly as one 

 passes outward into the peripheral field. The dotted line represents the acuity of vision in 

 dim lights. The fovea, it will be noticed, is less sensitive than the parts of the retina at an 

 angular distance of 10 or even 60. 



is the case, for instance, with the moon. Nevertheless, 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 in the peripheral field of vision the acuity of vision 

 diminishes at first very rapidly, so that at 20 degrees, for instance, 

 from the center of the fovea the physiological point on the retina is 

 0.035 mm.; that is, it has a diameter ten times as large as in the 



