194 The Animal Mind 



merely of the direction from which the greatest illumination 

 comes might produce this kind of an image, which would of 

 course not allow the perception of objects, only that of 

 brightness distribution. ( We have already seen that the orien- 

 tations of certain animals to light seem to be produced through 

 a tendency to take such a position that the two eyes shall be 

 equally illuminated. If the two visual fields are combined in 

 the case of such animals, as they are in our own binocular 

 vision, under ordinary conditions the oriented position would 

 give a visual field whose brightness is equal throughout, 

 while any other position would give greater brightness 

 at one side of the field. If they are not combined, if there 

 is no binocular vision, we cannot imagine what the resulting 

 perception is. That the direction from which the light comes 

 influences ants in finding their way is, we have seen, the opin- 

 ion of Lubbock (248 ) and of Turner (408). It was found not 

 to be important to white rats in learning a labyrinth path (43i).1 



76. Methods of investigating the Visual Image: the 

 Size Test 



The presence of a visual image that is something more than 

 a visual field of graded brightnesses has been tested by meth- 

 ods which may be divided into two groups : those which 

 investigate the effect of stimuli differing in area but of the 

 same intensity, and those which test discrimination of the 

 form of objects. 



Bohn's observations on the mollusk Littorina show that its 

 reactions are influenced by the size of the illuminated or dark- 

 ened surface, as well as by the intensity of the light. When 

 neither very wet nor very dry, Littorina will react to small 

 objects in its neighborhood, whereas in an extreme state of 

 " hydratation " or desiccation it responds to the attraction or 

 repulsion of the larger screens with fatal uniformity (55). 



