September 29, 1916] 



SCIENCE 



471 



lacking beyond fourteen hundred feet, accord- 

 ing to Gleichen. 1 



In the case of a man moving through a for- 

 est or any maze-like region, owing to the fron- 

 tal position of his eyes, the axis of his vision 

 is parallel to his motion; hence the apparent 

 displacement of the trees of the forest as seen 

 by him, due to his forward motion, is a mini- 

 mum. The effect is similar to that when a 

 person is riding on a railway and looking out 

 from a front window of a car straight at the 

 track and its immediate surroundings. It is 

 obvious that relative displacement of objects 

 near the track in the retinal picture due to 

 motion of the observer is very slight. In 

 frontal vision, as in man, a displacement of the 

 head sidewise affords a powerful means of 

 measurement of distance, as has been pointed 

 out, probably first by Helmholtz. 



The eyes of most birds, fish and reptiles are 

 so situated in the skull that only lateral vision 

 is possible. A number of species of birds, 

 however, have the ability to turn the eyes so 

 as to give binocular vision at will, and some 

 species have two distinct vision fovese, " yellow 

 spots," in each eye, for distinct sight for the 

 two types of vision. Other species have two of 

 the round and one streak-like foveas; particu- 

 larly certain ground-feeding birds of the snipe 

 family, etc. 2 Some of the mammalia have 

 lateral vision, some have their eyes so placed 

 that the retinal pictures partially overlap and 

 binocular vision is possible; others have fron- 

 tal vision as in man. It is interesting to con- 

 sider in what manner the lateral position of 

 the organs of vision enters into the determina- 

 tion, for example, by a bird, of the distances 

 of surrounding objects. It is an important 

 question, owing to the intimate connection of 

 such determinations with the " sense of direc- 

 tion" problem. 



There seems to be good evidence that those 

 living creatures 'having side vision have a de- 

 cided advantage over man in their ability to 

 gauge or measure the relative distances of 



i A. Gleichen, ' ' Die Theorie der Modernen 

 Optischen Instrumente, " p. 184. 



2 ' ' Lehrbueh der Vergleichenden mikroskopi- 

 schen Anatomie, ' ' Part 7, pp. 82-84. 



their surroundings, owing to the lateral posi- 

 tion of the eyes. "When a bird or a mammal, 

 etc., with its eyes so placed, moves forward, 

 the principal visual axis is perpendicular to 

 its motion. This gives the maximum apparent 

 displacement of objects with every forward 

 motion of the creature. 



This displacement of surrounding objects 

 gives to the animal having lateral vision a 

 means of a continual register of the relative 

 distances apart of surrounding objects. To 

 use the analogy of a person riding on a rail- 

 way, it is as if one was looking out of a side 

 window of a car. There is the additional ad- 

 vantage for the animal, since it may be said 

 to be looking out of two side windows, one on 

 each side. There is, moreover, the possibility, 

 in the case of creatures having lateral vision, 

 of a differential effect, since a change in the 

 direction of the head while in the forward mo- 

 tion would be registered on the retina of each 

 eye. The advantages of lateral vision for 

 measuring the relative distance of surround- 

 ing objects is illustrated as follows: 



The arrows in Figs. 1 and 2 are meant to 

 represent a man moving forward along a 

 straight path 15 feet, and a bird also moving 

 forward 15 feet respectively. Both observe 

 trees (the crosses) at 100 feet distance from 

 A, the starting-point. 



The trees are in front, nearly, of the man 

 in the one case, and at the side of the bird in 

 the other. The angles a and b are the angular 

 measures of apparent displacements in the two 

 cases, due to the forward movements. "With 

 the distances given in Figs. 1 and 2, these 

 angular measurements are as 5 is to 1 in favor 

 of the bird, but this is assuming that the man 

 has only one eye. For distances within a few 

 hundred feet of the man, as in Figs. 1 and 2, 

 binocular vision is a means of measurement of 

 distances, and hence the bird's advantage is 

 less than the ratio given. It is, in fact, hardly 

 possible to state a true measure of the ad- 



In Fig. 1 it is necessary to place the trees 

 considerably to one side of the man's path; 

 otherwise there would be no angular displace- 

 ment of the trees for the man whatever. 



