any small object, it will be seen single, because the two retinal images fall on corresponding or identical points, viz., on the two central spots. In Fig. 32 the two eyes, R and L, are turned directly on A. The image of this object will therefore fall on the central spots c c', and the object will be seen single. Objects at nearly the same distance, as for example D, a little to the right or left or a little above or below the point of sight, are also seen single; because the retinal images d and d' are on correspondent halves-i. e., the internal or nasal half of R and the external or temporal half of L—and at the same distance from the central spots c c', and therefore on identical points. Objects lying in a horizontal circle passing through the point of sight and the centers of the eyes, O O', are usually supposed to be seen single. This is nearly true, except when the point of sight is very near. This circle has been called the horopteric circle of Müller.

Objects, as already said, beyond or nearer than the point of sight, are always seen double. The reason is, that their retinal images always fall on non-corresponding points. This is shown in the diagram Fig. 33. While the two eyes, R and L, are fixed upon A, this object will be seen single, for its images, a and a', fall upon the central spots. But if, while still looking at A, we observe B and C, we shall see that both are double. The reason is, that the images of B, viz., b b', fall upon the two nasal or internal halves of the retina, which are non-corresponding; while the images of C, viz., c c', fall upon the two external or temporal halves of the retina, which are also non-corresponding. If the external double images be all referred to the plane of sight, PP (which, however, is not the fact), as is usually represented in diagrams, then the position of the dou

ble images will be correctly represented by cc', bb'. It is seen at a glance that the images cc' of Care heteronymous, while the images bb' of B are homonymous. Generally, all the field of view within the lines

of sight, A a, A a', belongs to the temporal halves of the retina, while all outside of these lines belongs to the nasal halves. Or, again, double images formed by impressions on the two nasal halves of the retina are homonymous, while those formed by impressions on the two temporal halves are heteronymous.

Definition of Horopter.-We have seen that the object at the point of sight is seen single; and all objects at the same or nearly the same distance, but a little to the right or left, or above or below, are also either seen single, or else the doubling, if any, is usually imperceptible. On the contrary, all objects farther or nearer than the point of sight are seen double. Now the surface of single vision—i. e., the surface passing through the point of sight, all the objects lying in which are seen single-is called the horopter. Whether there is such a surface at all, and if there is, what is its form, are questions upon which the acutest observers differ. Some have made it a plane, some a spherical surface. Some, by purely geometrical methods, have given it the most curious forms and properties; while others, by purely experimental methods, have come to the conclusion that it is not a surface at all, but a line. We are not now prepared to discuss this question, but shall return and devote to it a special chapter.

Supposed Relation of the Optic Chiasm to the Law of Corresponding Points.-In the optic chiasm, Fig. 20, page 54, there is certainly a partial (but only a partial) crossing of the fibers of the two optic nerves. Many physiologists connect this fact with this remarkable law. There is probably such a connection. But But many go farther. They think that some of the fibers of each optic nerve cross over to the other eye, and some do not; and that those which cross over supply the internal or nasal halves, and those which do not cross over supply the temporal halves. Thus, in the diagram Fig. 34, the fibers of the right optic nerve-root O, as it comes from the brain, go to supply the temporal half t of the right retina, and, by crossing, the nasal half n' of the left retina, and these are corresponding halves. So also the

fibers of the left optic nerve-root O' go to supply the temporal half t of the left and nasal half n of the right retina. Still further, they think that the fibers coming from corresponding or identical points, or rods, or cones

O O', optic roots; N N', optic nerves; R and L, sections of the two eyes; c c', central spots; n n', the nasal halves, and t t', the temporal halves, of the retina.

in the two retinæ are not only thus carried by the same optic root, but finally unite to form one fiber, or at least terminate centrally in one brain-cell, and thus form one single sense-impression. It is almost needless to say that, while this is an interesting speculation, it is nothing more; for the supposed union of fibers from corresponding rods or cones can probably never be either proved or disproved.

Theories of the Origin of this Law.-The perception of direction and the correspondence of retinal and spatial points are certainly inherent properties of the retina, being connected with its structure. The formeri. e., the perception of direction-we have seen, is a general property of sensory nerves, only developed into mathematical accuracy in the case of the optic nerve;

the latter-i. e., the correspondence of retinal and spatial points-is only the expression of this mathematical accuracy of perception of direction; and both are connected with the structure of the bacillary layer. Undoubtedly, then, this property is innate and antecedent to all experience. What the infant learns by experience is not direction, but distance and size of the object. Direction is a primary datum of sense. But the property of corresponding points of the two retina and of identical spatial points in the two fields of view seems to be less absolutely simple and primary. The questions, "Is this property innate, instinctive, antecedent to experience? or is it wholly the result of experience?” have been long and hotly disputed by the profoundest thinkers on this subject. The former view has been held by Müller, Pictet, and others; the latter by Helmholtz, Brücke, Prévost, and Giraud Teulon: the one is called the nativistic, the other the empiristic theory.

We shall not follow the history of this dispute, nor detail the arguments brought forward on each side; for the tendency of modern science, under the guidance of the theory of evolution, is to bring these two opposite views together, and reconcile them by showing that they are both in a degree true, and therefore not wholly inconsistent with each other. The difficulty heretofore has been that anatomists and physiologists have studied man too much apart from other animals, and thus the amount of inherited, innate, instinctive qualities has been greatly underestimated by some and overestimated by others. A new-born chicken, in a few minutes after breaking the egg-shell, will see an object, direct the eyes upon it, walk straight up to it, and seize it. Evidently there is in this case not only a perception of direction, antecedent to all experience, but also some