472 



SCIENCE 



[N. S. Vol. XLIV. No. 1135 



Frontal Viaion, ( Man) 



STree 



Lateral Vision , ( Bird ) 



I 



Man 



Bird 



Tree Tree 



Figs. 1 and 2. These diagrams show the parallax (displacement) of objects as viewed in frontal 

 vision and in lateral vision. The ratio of o to a, with distances given in the text, is 5 to 1. 



In order to give figures showing the ad- 

 vantage of lateral vision over frontal vision 

 at greater distances due to motions forward, 

 two cases were selected where the objects are 

 two trees, one at 1,000 feet and the other 2,000 

 feet distant, respectively, from the observer 

 (man and bird), the trees being approximately 

 in the axis of vision in each case. To obtain 

 any values it is necessary (as in Fig. 1) to 

 place the two objects observed (trees) out of 

 line of the direction of the man's motion ; that 

 is, to one side of his path. The results in Fig. 

 3 and Fig. 4 were obtained from a graphical 

 construction which is omitted. 



Fig. 3 is an equal parallax curve and it 

 shows the distance that a man and bird must 

 each move forward to give the same apparent 

 displacement of trees against the horizon. In 

 the figure the points are plotted as they were 

 found from the graphical construction, and 

 show a slight irregularity. 



Fig. 4 is a curve constructed from Fig. 3, 

 and it illustrates the decided advantage of the 

 bird over the man. 



The bird's advantage is obtained by divid- 

 ing the distance that the man must move 

 forward by the distance that the bird moves 

 forward to obtain equal displacement of the 

 objects viewed. For example, at the point M, 

 the man has moved forward 20 feet and the 

 bird's advantage is 12 to 1. At point N the 

 man has moved forty feet, and the bird's ad- 

 vantage is 10 to 1, etc. 



The distances of one thousand and two thou- 

 sand feet were taken as a basis for the curves 

 in Figs. 3 and 4 because binocular vision is 

 not a means of measurement at these dis- 

 tances, and hence the advantages for the bird 

 are as they have been given in the figures. 



This method of demonstrating the advantage 

 of lateral vision serves chiefly to give some 

 numerical expression of the value of that type 

 of vision for the measurement of the distances 

 apart of objects in the field of view. 



In conclusion it may be stated then that an 

 animal having only lateral vision, if at rest, 

 has no means of measuring the relative dis- 

 tance of surrounding objects, except by com- 

 parison of the various size of objects, and 



