1 1 32 VISION. 



Using the method of binocular combination, Dixon x found that the 

 variation in the angle of convergence which he could make without 

 altering his accommodation, decreased with the distance of the objects. 

 At 100 cm. he found that he could vary his convergence to an amount 

 corresponding to 6 D, without alteration of accommodation. 



How far the association between convergence and accommodation 

 persists in monocular vision, has not been satisfactorily determined. 



The axes of the eyeballs during sleep are usually described, 

 following J. Miiller, as directed upwards and inwards. Eaehlmann and 

 Witkowski, 2 however, found that any position might occur, and that 

 inco-ordinated movements of a special character were common during 

 sleep. The movements were very slow, and might lead to any kind of 

 divergence, horizontal or vertical, of the lines of vision. They also 

 found that movement of one eye only, occurred frequently. Henke 3 and 

 others have found that while going to sleep crossed and slanting double 

 images may be observed, indicating divergence and swivel rotation of 

 such a kind that the vertical meridians converge upwards. 



As already mentioned, co-ordinated movements persist in the blind, 

 even in congenital blindness, although in some cases minor degrees of 

 inco-ordination have been observed after many years. 



In newly-born children, most observers have failed to observe 

 co-ordinated eye movements till some time after birth. Eaehlmann and 

 Witkowski i found the movements often inco-ordinated and much like 

 those in sleep, but more rapid. They failed to observe movements of 

 fixation in the first ten days. Two such excellent observers as Bonders 5 

 and Engelmann, however, observed definite binocular fixation with altera- 

 tion of convergence in a child a few minutes after birth, but Hering has 

 pointed out that this result can only be expected exceptionally, since 

 most children pass the early days of life almost in a condition of sleep. 



These various cases have all been discussed in connection with the 

 question whether the association of eye movements is innate or acquired, 

 a question important in connection with theories of the genesis of 

 spatial perception. The observations of Donders are sufficient evidence 

 that the association is not acquired in the life of the individual, but it 

 is also certain that the association is not so fixed as to be incapable of 

 being dissolved under many conditions. 



Spatial Peeception. 



Visual localisation.— This has been referred to factors of three 

 kinds, — to sensations derived from movements of the eyeballs and of 

 accommodation ; to sensations, or modifications of sensations, arising 

 from the simultaneous stimulation of the two retinae ; and, thirdly, to a 

 number of factors of a more purely psychological nature. The sensations 

 of movement have been referred to peripheral changes set up by move- 

 ments of the eyeballs, and also to central changes connected with 

 innervation. The psychological factors are of great importance, and in 

 vision with one eye and at great distances are the chief or only means 

 of localisation. They also, however, influence the process of localisation 

 under all ordinary conditions, and in order to investigate the physio- 

 logical factors they must be excluded. This is done by using very fine 



1 Mind, London and Edinburgh, 1895, vol. iv. p. 205. 



- Arch./. Physiol., Leipzig, 1877, S. 454. 3 Arch.f. Ophth., 1864, Bd. x. Abth. 2, S. 181. 



4 Loc. cit., p. 459. 5 Arch./. Ophth., 1871, Bd. xvii. Abth. 2, S. 34. 



