*Binocular Vision.*—In addition to directing the eyeballs so that light may enter them to the best advantage from different objects, the 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 accomplished 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 sensibility to light. *Visual Perceptions.*—"Seeing" is very largely the mental interpretation 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 interpretation 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 direction 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.