Page:Mind (Old Series) Volume 12.djvu/358

THE PERCEPTION OF SPACE, (ill.) 345 moving object in it ; in the second, the total field swimming more or less steadily in one direction ; in the third, a sudden jump or twist of the same total field. The feelings of convergence of the eyeballs permit of the same ambiguous interpretation. When objects are near we converge strongly upon them in order to see them ; when far, we set our optic axes parallel. But the exact degree of convergence fails to be felt ; or rather, being felt, fails to tell us the absolute distance of the object we are regarding. Wheatstone arranged his stereoscope in such a way that the size of the retinal images might change without the con- vergence altering ; or conversely, the convergence might change without the retinal image altering. Under these circumstances, he says, 1 the object seemed to approach or recede in the first case, without altering its size ; in the second, to change its size without altering its distance just the reverse of what might have been expected. Wheatstone adds, however, that " fixing the attention " converted each of these perceptions into its opposite. The same perplexity occurs in looking through prismatic glasses, which alter the eyes' convergence. We cannot decide whether the object has come nearer, or grown larger, or both, or neither ; and our judgment vacillates in the most surprising way. We may even make our eyes diverge, and the object will none the less appear at a finite distance. When we look through the stereoscope, the picture seems at no determinate distance. These and other facts have led Helmholtz to deny that the feeling of convergence has any very exact value as a distance- measurer. With the feelings of accommodation it is very much the same. Donders has shown 2 that the apparent magnifying power of spectacles of moderate convexity hardly depends at all upon their enlargement of the retinal image, but rather on the relaxation they permit of the muscle of accommoda- tion. This suggests an object farther off, and consequently a much larger one, since its retinal size rather increases than diminishes. But in this case the same vacillation of judg- ment as in the previously mentioned case of convergence takes place. The recession made the object seem larger, but the apparent growth in size of the object now makes it look as if it came nearer instead of receding. The effect thus contradicts its own cause. Everyone is conscious, on 1 Philosophical Transactions, 1852, p. 4. 2 Anomalies of Accommodation and Refraction (New Sydenham Soc. Transl.), London, 1864, p. 155.