THE PERCEPTION OF SPACE. 235 



the reverse of Avhat 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 w^iether 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 dis- 

 tance. 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 f 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 

 judgment as in the previously mentioned case of converg- 

 ence 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 first putting on a pair of spectacles, of a doubt whether 

 the field of view draws near or retreats. :]: 



There is still another deception, occurring in persons ivho 

 have had one eye-muscle suddenly paralyzed. This deception 



* Physiol. Optik, 649-664. Later this author is led to value converg- 

 ence more highly. Arch. f. (Anat. u.) Physiol. (1878), p. 322. 



f Anomalies of Accommodation and Refraction (New Sydenham Soc. 

 Transl., London, 1864), p. 155. 



J These strange contradictions have been called by Aubert ' secondary ' 

 deceptions of judgment. See Grundzlige d. Physiologischen Optik (Leip- 

 zig, 1876), pp. 601, 615, 627. One of the best examples of them is the small 

 size of the moon astirst seen through a telescope. It is larger and brighter, 

 so we see its details more distinctly and judge it nearer. But because we 

 judge it so much nearer we think it must have grown smaller. Of. Char- 

 pentier in Jahresbericht, x. 430. 



