spatially Determined Reaction 241 



Convergence, the turning of the eyes toward each other 

 to bring the two images of an object on the central part 

 of the retinas, which is an important aid to human estima- 

 tions of distance, is also necessarily lacking in animals 

 without binocular vision. A third factor in our own per- 

 ceptions of distance, the accommodation of the crystalline 

 lens, that is, the alteration of its convexity through the 

 pull of the accommodation muscle to enable it to focus 

 objects at different distances, has been carefully studied 

 in connection with the lower animals by Beer^ Through 

 experiments on the refractive powers of eyes dissected 

 from the dead animal, he reached the conclusion that no 

 invertebrates but cephalopods have the power of accom- 

 modation. It is rudimentary or lacking also in some mem- 

 bers of the fish, lizard, crocodile, snake, and mammal 

 families. In cephalopods, fishes, amphibians, and most 

 reptiles, the process of accommodation does not involve a 

 change in the form of the lens, but an alteration in the dis- 

 tance between the lens and the retina. The device of 

 increasing the curvature of the lens for vision of near 

 objects appears first in certain snakes, and is found through- 

 out the higher vertebrates {■^t,, 34, 35, 37). 



Where accommodation does not exist, as in most in- 

 vertebrates, it is possible to trace other arrangements for 

 adapting vision to the distance of the object seen. Thus 

 in compound eyes, part of the eye may be adapted to near 

 vision and part to far vision. This is suggested by the 

 fact that some of the little tubes, or ommatidea, of which 

 the compound eye is composed, diverge from each other 

 by a less angle than others, indicating that they are suited 

 to the reception of more nearly parallel rays. In insects 

 with both simple and compound eyes one form may be 

 used for near and one for far vision. It has been main- 



