382 COORDINATION AND SENSATION 



muscles also enable two eyes to be used as one. Whenever the eyes 

 are directed toward the same object, an image of this object is formed 

 on the retina of each. Double vision is prevented only by having the 

 images fall on corresponding places in the two eyes. This is accom- 

 plished by the muscles. In each act of seeing, it becomes the task 

 of the superior and inferior recti muscles to keep the eyes in the 

 same plane, and of the external and internal recti muscles to give 

 just the right amount of convergence. If slight pressure is exerted 

 against one of the eyes, the action of the muscles is interfered with 

 and, as a consequence, one sees double. The advantages of two 

 eyes over one in seeing lie in the greater distinctness and broader 

 range of vision and in the greater correctness of judgments of 

 distance. 



Visual Sensations. The visual sensations include those of color 

 and those of a general sensibility to light. Proof of the existence of 

 these types of sensation is found in color blindness, a defect which 

 renders the individual unable to distinguish certain colors when he is 

 still able to see objects. Color sensations are the results of light waves 

 of different lengths acting on the retina. While the method by which 

 waves of one length produce one kind of sensation and those of another 

 length a different sensation is not understood, the cones appear to be the 

 portions of the retina acted on to produce the color. On the other 

 hand, the rods are sensitive to all wave lengths and give general sensi- 

 bility to light. 



Visual Perceptions. " Seeing " is very largely the mental interpreta- 

 tion of the primary sensations and the conditions under which they 

 occur. For example, our ability to see objects in their natural positions 

 when their images are inverted on the retina is explained by the fact 

 that we are not conscious of the retinal image, but of the mind's inter- 

 pretation of it through experience. Experience has also taught us to 

 locate objects in the direction toward which it is necessary to turn the 

 eyes in order to see them. In other words, we see objects in the direc- 

 tion from which the light enters the eyes. That the object is not always 

 in that direction is shown by the image in the mirror. The apparent 

 size and form of objects are inferences, and they are based in part 

 upon the size and form of the area of the retina stimulated. We judge 

 of distance by the effort required to converge the eyes upon the objects, 

 by the amount of divergence of the waves entering the pupil, and also 

 by the apparent size of the object. 



