126 



BIOPHYSICAL CHARACTERISTICS OF THE EYE 



of measurement are used, as in the estimation of the position of a slide 

 rule, or of a meniscus of a barometer or chemical burette, the observa- 

 tions are much less accurate than those obtained by a coincidence 

 method as is used in mensuration. 



Variation of Acuity with Distance from Fovea 



The acuity of vision rapidly decreases as the image moves away from 

 the fovea. Figure III— 15 shows this decrease in visual acuity and its 

 variation with angular departure from perpendicular incidence on the 



1.0 



0.9 



0.8 



IT 



Temporal 



40 



30 



20 



30 



40 



10 10 



Degrees from visual axis 



Fig. Ill— 15. Variation of acuity with distance from the fovea centralis in direct 

 vision. Composite curve after Helmholtz, Handbuch der physiologischen Optik. 



fovea. At 5° from the fovea it falls to one third of its maximum. The 

 broken line represents the visual acuity under very low intensities of 

 illumination. On the other hand, when the eyes are used for fine work 

 at high-level brightness, the visual acuity is very high at the center of 

 the fovea. The drop in visual acuity across the fovea at low intensities, 

 apparent in the depression of the dotted curve, indicates why we see. a 

 faint star in the night sky better near the rim of vision than when it is 

 focused on the fovea. The crossing of the blind spot is shown by the 

 gap in the two lines at the nasal side of the axis. 



