357. =Single Vision.=--Whenever we see any object with both eyes there is an image formed in each eye, and impressions go from both eyes by the optic nerves to the brain. And yet with these two impressions there is no double vision so long as the two eyes correspond with each other in situation. This is because the image in one eye occupies the same place on the retina that the image in the other eye does. The correspondence is ordinarily perfect, the two eyes turning always together in the same way, upward, downward, or laterally, without the least variation. You can observe the effect of a want of this correspondence by pressing one of the eyes in some direction with the finger while the other is left free to move in obedience to the muscles. When this is done every object appears double, because its image occupies in one eye a different part of the retina from what it does in the other, and so two different impressions are carried to the brain. The same thing occurs in squinting, in which the action of the muscles of the two eyes does not agree. Ordinarily in squinting there is not double vision, because the mind has the habit of disregarding the impressions that come from the defective eye. But when squinting occurs suddenly from disease there is double vision, for it takes a little time to form the habit referred to. 358. =Stereoscope.=--The images of objects in the two eyes, though always similar, are not generally perfectly alike. They are so only when the object presents a simple surface, as in the case of pictures. When the object presents two or more surfaces to the sight the images are more or less unlike. This can be illustrated in a very simple way. Hold a book up straight before your eyes with its back toward you. You see the back and both sides. Now if you shut your right eye you will see with the left the back of the book and the left side. That is, these two parts of the book are imaged on the retina of the left eye. By shutting the left eye it will appear that the image in the right is different, for you see now with the back the right side of the book. Here you have the explanation of the stereoscope. In the right side of this instrument you have the picture of the object as the object itself would appear to the right eye, and in the left side you have the picture of it as it would appear to the left eye. Thus, if a book in the position alluded to above were the object, in the right picture there should be represented the back together with the right side of the cover, and in the left the back with the left side of the cover. The two impressions, carried to the brain by the optic nerves, give together the impression of a solid book. The same principles apply to the representation of all solids in the stereoscope. [Illustration: Fig. 249.]