=Double Images.=--Now it is an immediate consequence of the law of identical location of images falling on geometrically similar points that _images which fall upon geometrically_ DISPARATE _points of the two retinæ should be seen in_ DISPARATE _directions, and that their objects should consequently appear in_ TWO _places, or_ LOOK DOUBLE. Take the parallel rays from a star falling upon two eyes which converge upon a near object, _O_, instead of being parallel as in the previously instanced case. The two foveæ will receive the images of _O_, which therefore will look single. If then _SL_ and _SR_ in Fig. 10 be the parallel rays, each of them will fall upon the nasal half of the retina which it strikes. But the two nasal halves are disparate, geometrically _symmetrical_, not geometrically _similar_. The star's image on the left eye will therefore appear as if lying to the left of _O_; its image on the right eye will appear to the right of this point. The star will, in short, be seen double--'homonymously' double. Conversely, if the star be looked at directly with parallel axes, any near object like _O_ will be seen double, because its images will affect the outer or cheek halves of the two retinæ, 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 positions seen will be less widely disparate than in the other case, and the double images will 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 retinæ. For the detail of these one must consult the special treatises.