September 29, 1916] 



SCIENCE 



473 



ft Man's Distance From A 



Pig. 3. Equal parallax curve, showing the distances a man and a bird must move forward to 

 e the same apparent displacement of objects against the horizon. 



Man's Distance From A (fcet) 



Fig. 4. Advantage curve. The bird's advantage is obtained by dividing the distance that the 

 man must move forward by the distance that the bird moves forward to obtain the same displace- 

 ment of objects viewed. At M, the advantage is 12 to 1; at N, 10 to 1, etc. 



through accommodation, etc. An animal hav- 

 ing binocular vision (man, ape, cat, etc.), if at 

 rest, has such a means for the measurement of 

 objects that are not more than a few hundred 

 feet distant. 



If creatures having each type of vision are 



moving forward, then there is a distinct ad- 

 vantage in favor of the one having lateral 

 vision. This is especially the case if the ob- 

 jects viewed are in the middle ground (500 to 

 1,500 feet away), or at greater distances, since 

 binocular vision furnishes little, if any, means 



