1 148 VISION. 



is moved in the same way, as in rinsing out a basin, the figure appears to 

 rotate round its centre. One of the factors upon which this depends 

 appears to be the unequal distinctness of vision in different meridians, 

 due to astigmatism. If a number of such figures be placed round a 

 ring with teeth projecting inwards, and the whole moved in the same 

 way, the inner toothed ring will appear to rotate in the opposite direc- 

 tion to that of the figures, and more slowly. 



A peculiar illusion of movement was first described by Charpentier, 1 

 and later by Aubert 2 and Exner ; 3 by the latter it is referred to " auto- 

 kinetic " sensations. If a small feebly illuminated object be observed in 

 a room, otherwise completely dark, the object will appear to move with 

 a certain rapidity in a definite direction. Charpentier found its angular 

 velocity about 2° to 3° per second, and that the total movement might 

 be through as much as 30°. He found that the appearance was not due 

 to unconscious movement of the eyes, and occurred, though in a less 

 marked degree, when other control points of light were present. Its 

 direction was influenced by the idea of a certain direction ; a loud sound 

 caused a movement towards the supposed direction of the sound, and 

 direction of the attention to the floor caused movement downwards, to 

 the roof upwards. If due to movement of the eyes, the apparent move- 

 ment should have been in the opposite direction. Aubert found also 

 that a real movement of the object might not be seen. 



Localisation in animals. — The question of the mechanism of the 

 estimation of space relations in animals below man is one of great 

 difficulty. One can hardly believe that the more psychological motives of 

 localisation, which play so important a part in man, can be of any importance 

 here ; on the other hand, the most definite physiological factor, the binocular 

 mechanism, is out of the question in some animals, as birds, and must be of 

 a special nature in other animals. Only man and some primates have the two 

 features, optic axes capable of convergence and a single central fovea. Other 

 animals have structural peculiarities which have been regarded as of a foveal 

 nature, but we know nothing of their function. It is quite possible that 

 binocular mechanisms may exist wholly different from that found in man. 

 Loeb 4 has suggested that the marked astigmatism found in many animals may 

 play a part in space perception. The astigmatism is so considerable that the 

 form of the retinal image must be appreciably affected by movements of an 

 object nearer to or away from the eye, and it is quite conceivable that these 

 changes in form might act as the basis of localisation. 



1 Compt. rend. Acad. d. sc, Paris, 1886, tome cii. pp. 1155 and 1462. 



2 Arch.f. d. ges. Physiol., Bonn, 1887, Bd. xl. S. 477 and 623. 



3 Ztschr. f. Psychol, u. Physiol, d. Sinncsorg., Hamburg u. Leipzig, 1896, Bd. xii. S. 313. 



4 Arch.f. d. ges. Physiol., Bonn, 1887, Bd. xli. S. 371. 





