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. 22, page 51, there is certainly a partial (but only a partial) crossing of the fibers of the two optic nerves. Many

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

physiologists connect this fact with this remarkable law. There is probably such a connection. 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 these which do not cross over supply the temporal halves. Thus, in the diagram Fig. 41, the fibers of the right optic nerve-root 0, as it comes from the brain, go in part directly to supply the temporal half t of the right retina, and in part 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 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 individual experience.* What the infant learns by experience is not direction, but distance and size of the object. Direction is a primary datum of sense (page 73). 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 question, "Is this property innate, in

*It is probably the result of experience, but of ancestral experience, inherited by the individual.

stinctive, antecedent to experience? or is it wholly the result of experience?" has 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 perception of distance, and the wonderful coördination of muscles necessary for standing and walking, and directing the movements of the eyes. A young rumi

nant animal in a few minutes after birth will stand and walk, and direct its motions by sight. A bird of wild species, hatched in a cage and kept in a cage until it is fully fledged and its muscles are sufficiently developed, if then thrown into the air, will fly away with ease, although the coördination of many muscles in the act of flying is something so marvelous that it could not be learned in a lifetime of trial, unaided by inherited

capacity. Inherited powers are still more marvelous in the case of insects.

Manifestly, then, the wealth of capacities in all directions possessed by the individual is partly inherited and partly acquired by individual experience. In animals the inherited, in man the individually acquired, wealth predominates. But all wealth is acquired. Even that inherited is ancestral experience accumulated and transmitted by the law of heredity. Even instinct is "inherited experience." Thus, then, it is evident that the property of corresponding points of the two retina, and therefore of identical points in space, is partly inherited and partly acquired by individual experience. It is doubtless wholly the result of experience, but not wholly of individual experience.

Consensual Adjustments.-There are therefore two adjustments of the eye in every voluntary act of sight, viz., focal and axial. In the former, each eye is adjusted by the ciliary muscle to make a perfect image on the retina; in the latter, the two eyes are turned by the recti muscles so that their axes shall meet on the point of sight, and the images of the object looked at shall fill on the central spots. The one is an adjustment for distinct vision, the other for single vision. There is associated with these still a third adjustment, but of far less importance, viz., the adjustment of the pupil. The pupil contracts and expands not only as the light is bright or faint, but also as the object is near or far. These three adjustments take place together and without distinct volition for each-i. e., by the one voluntary act of looking. They are therefore consensual movements, and usually regarded as indissolubly associated. We shall show hereafter that under certain circumstances they may be dissociated.

The Two Fundamental Laws.-There are also two great and fundamental laws by which all visual phenomena are explained, viz., the law of direction and the law of corresponding points. The one gives the true position of all points in space, and therefore entirely explains the apparent anomaly of erect vision with inverted retinal images; the other gives coincidence of the spatial representative of corresponding points in the two fields of view, and therefore entirely explains the second anomaly of vision, viz., of single vision with two retinal images. Both may in fact be called laws of corresponding points. The one

asserts the correspondence point for point of retinal rods and cones with external space, with ray-lines connecting and crossing in the nodal point; the other asserts a correspondence point for point of the rods and cones of the two retina, and the coincidence of their representatives in the two fields of view. From the one law flow all the phenomena of monocular, from the other all the phenomena of binocular vision. But underlying both of these is the still more fundamental law of external projection of retinal states.

All the phenomena of binocular vision are explained by the law of corresponding points. But the phenomena are so numerous, so illusory, and so difficult of analysis, that the connection is by no means obvious. The science of binocular vision consists in tracing this connection, and thus explaining the phenomena. It will be our object, then, to take up all the most important phenomena of binocular vision, and explain them in this way.