NOCTURNALITY AND VISUAL ACUITY 207 



The diurnal animal, because he is cone-rich, has an acuity of vision 

 which makes the eye his best sensory instrument; but the nocturnal form, 

 being cone-poor, has unsharp vision and can make more accurate identi- 

 fications of enemies and food with his nose than with his eyes. The 

 'minimum separabile for parallel lines — the angular distance they must 

 be apart to be seen as separate — has been determined experimentally for 

 a number of animals by various investigators. Some of the values ob- 

 tained, not necessarily at all close to maximal and minimal values for all 

 vertebrates, are listed in Table V. 



Table V 



VISUAL ACUITIES FOR PARALLEL LINES (From various sources) 



, . , Visual Corresponding distance Visual angle corresponding 



Diurnal animals: angle, on retina, to imm. distance 



minutes micra along visual cortex 



Human adult 0.44 L89 



0.48 2.06 



0.50 2.14 



(different reports) 0.80 3.43 



0.82 3.52 



0.83 3.56 



Child 0.62 2.67 



Chimpanzee 0.47 1.86 



Rhesus monkey 0.67 2.33 



Rhesus monkey, 



along visual axis 4' 



Rhesus monkey, 



7° from visual axis 20' 



Cebus monkey 0.95 3.31 



Pigeon 2.70 4.89 



Pigeon, 'homer' 0.38 .69 



Gamecock (no fovea) 4.07 9.58 



Nocturnal animals: 



Cat, along visual axis 5.5 1° 



Cat, 30° below axis 5° 



Alligator 11.0 



Opossum 11.0 



Rat, pigmented 26.0 23.8 



Rat, albino 52.0 47.7 



