Page:Mind (Old Series) Volume 12.djvu/350

THE PEECEPTION OF SPACE, (ill.) 337 The star will in short be seen double, ' homonymously ' double. Conversely if the star be looked at directly with parallel axes, will be seen double, because its images will affect the outer or cheek halves of the two retinae, instead of one outer and one nasal half. The position of the images will here be reversed from that of the previous case. The right eye's image will now appear to the left, the left eye's to the right the double images will be ' heteronymous '. The same reasoning and the same result ought to apply where the object's place with respect to the direction of the two optic axes is such as to make its images fall not on non-similar retinal halves but on non-similar parts of similar halves. Here of course the directions of projection will be less widely disparate than in the other case, and the double images should appear to lie less widely apart. Careful experiments made by many observers according to the so-called haploscopic method confirm this law and show that corresponding points, of single visual direction, exist upon the two retinae. For the detail of these one must consult the special treatises. Note now an important consequence. If we take a stationary object and allow the eyes to vary their direction and convergence, a purely geometrical study will show that there will be some positions in which its two images impress corresponding retinal points, but more in which they im- press disparate points. The former constitute the so-called horopter, and their discovery has been attended with great mathematical difficulty. Objects or parts of objects which lie in the eyes' horopter at any given time cannot appear double. Objects lying out of the horopter would seem, if the theory of identical points were strictly true, necessarily and always to appear double. Here comes the first great conflict of the identity-theory with experience. Were the theory true, we ought all to have an intuitive knowledge of the horopter as the line of distinctest vision. Objects placed elsewhere ought to seem, if not actually double, at least blurred. And yet no living man makes any such distinction between the parts of his field of vision. To most of us the whole field appears single, and it is only by rare accident or by special education that we ever catch a glimpse of a double image. In 1838, Wheatstone, in his truly classical memoir on binocular vision and the stereoscope, 1 showed that the disparateness of the 1 This essay, published in the Philosophical Transactions, contains the germ of almost all the methods applied since to the study of optical percep- 22