Page:Mind (Old Series) Volume 12.djvu/354

 THE PERCEPTION OF SPACE, (ill.) 341 direction MM and NN to the two retinae. If N be judged as far as O, it must necessarily lie where the two lines of visible direction NN intersect the plane of the arrow, or in two places, at N' and at N". If M be judged as near as 0, it must for the same reason form two images at M' and M". It is, as a matter of fact, true that we often misjudge the distance in the way alleged. If the reader will hold his fore- fingers, one beyond the other, in the median line, and fixate them alternately, he will see the one not looked at, double ; and he will also notice that it appears nearer to the plane of the one looked at, whichever the latter may be, than it really is. Its changes of apparent size as the convergence of the eyes alter, also prove the change of apparent distance. The distance at which the axes converge seems, in fact, to exert a sort of attraction upon objects situated elsewhere. Being the distance of which we are most acutely sensible, it invades, so to speak, the whole field of our perception. If two half- dollars be laid on the table a few inches apart, and the eyes fixate steadily the point of a pen held in the median line at varying distances between the coins and the face, there will come a distance at which the pen stands between the left half-dollar and the right eye, and the right half-dollar and the left eye. The two half-dollars will then coalesce into one ; and this one will show its apparent approach to the pen-point by seeming suddenly much reduced in size. 1 Yet, in spite of this tendency to inaccuracy, we are never actually mistaken about the half-dollar being behind the pen-point. It may not seem far enough off, but still it is farther than the point. In general it may be said that where the objects are known to us, no such illusion of distance occurs in any one as the theory would require. And in some observers, Hering for example, it seems hardly to occur at all. If I look into infinite distance and get my finger in double images, they do not seem infinitely far off. To make objects at different distances seem equi-distant, careful pre- cautions must be taken to have them alike in appearance, and to exclude all outward reasons for ascribing to the one a different location from that ascribed to the other. Thus Bonders tries to prove the law of projection by taking two similar electric sparks, one behind the other on a dark ground, one seen double ; or an iron rod placed so near to the eyes that its double images seem as broad as that of a fixated stove-pipe, the top and bottom of the objects being 1 Naturally it takes a smaller object at a less distance to cover by its image a constant amount of retinal surface.