Photoreception 



399 



TABLE 63. VISUAL ACUITIES FOR PARALLEL LINES (From various sources) 



Diurnal animals 

 Human adult 



(different reports) 



Visual 



angle, 



minutes 



0.44 

 0.48 

 0.50 

 0.80 

 0.82 

 0.83 



Corresponding distance Visual angle corresponding 

 on retina, to 1 mm. distance 



micra along visual cortex 



1.89 

 2.06 

 2.14 

 3.43 

 3.52 

 3.56 

 2.67 

 1.86 

 2.33 



3.31 



4.89 

 .69 



9.58 



23.8 



47.7 



4' 

 20' 



ness, color, position, motion, and distance. We are aware of these qualities 

 by virtue of the retinal image which is translated on the basis of experience 

 into what we consider to be the "true" size, shape, and position of the ob- 

 ject. The special adaptations which permit enhancement of the perception 

 of some of the abo\'e characteristics are as follows: 



Accommodation and Its Substitutes. Accommodation, i.e., focusing of the 

 image, may be brought about in various ways. In lampreys, teleosts, and 

 probably holosteans, the active process of accommodation moves the lens 

 backward to permit focusing on distant objects. In elasmobranchs, amphib- 

 ians, and snakes, the lens is activelv moved forward to adjust for near ob- 

 jects; and in mammals, birds, and reptiles other than snakes the curvature of 

 the lens is increased in order to focus on near objects. 



Visual Angles and Fields. The visual angle through which single vertebrate 

 eyes can see is generally about 170 degrees, with a lower extreme of 110 de 

 grees for certain air-living animals (owls) and still less for deep-sea fishes, 

 and a maximum of 200 degrees or more for a cat eve. The visual angle of a 

 human eve is 150 degrees. 



The position of the eyes in the head determines the extent of the binocular 

 field, if present. A few vertebrates (lamprevs, hammerhead sharks, a few 

 large-headed teleosts, the amphibian Cryptohranchtis, penguins of the genus 

 Spheniscus, and the larger whales) have no binocular field of vision; i.e., 

 both eyes can never focus on the same object at the same time. In most fishes 



