SPA TIAL PERCEPTION. 1133 



lines or points or falling objects, in which the factor of size is reduced to 

 a minimum, or is unknown. Fine suspended threads are better than lines 

 on paper, since in the latter case other markings or inequalities of the 

 paper surface may be of influence. 



Two kinds of localisation must be rigorously distinguished from one 

 another, namely, localisation relative to the fixation point, and localisation 

 of the fixation point itself ; or, more strictly, of the fixation point relative to 

 the individual. The apparent position of a fixed point does not necessarily 

 correspond with its actual position, and it would be more accurate to 

 follow Hering, and to speak of localisation relative to the nucleus or central 

 point (Kernpunkt) of visual space and localisation of the nucleus itself. 

 The surface containing the nucleus which appears parallel to the 

 plane of the face is called by Hering the nuclear plane (Kernflache), and 

 corresponds to the horopter of actual space. Since, however, fixation 

 point and nucleus coincide with one another under ordinary conditions, 

 the former designation may be employed. 



Localisation relative to the fixation point. A point may be 

 localised on either side of, above or below, and nearer or farther 

 than the fixation point. The three dimensions may be termed breadth, 



FIG. 407. Greeff. 



height, and depth respectively. It is the third which has been most 

 fully investigated, and about which there is most difference of opinion. 

 It is a familiar experiment that the relative distances of two objects 

 (needle and thread) is estimated much better with two eyes than with 

 one, and the great importance of binocular vision is shown most con- 

 clusively by means of Bering's fall experiment. 1 An observer looks 

 through a broad tube at a bead on a vertical thread which acts as the 

 fixation point. The psychological factors are excluded by using falling 

 balls of different colours and unknown size. It is found that, with only 

 one eye open, the observer is unable to distinguish whether the falling 

 ball is nearer or farther than the bead, while with two eyes the accuracy 

 of perception is very considerable, the just perceptible difference of 

 distance being about T V of the distance of the fixation point. It has 

 been objected, that perception might depend on some other factors, 

 such as the apparent rate of movement of the ball. In order to over- 

 come this and other objections, Greeff 2 has modified the apparatus, as 

 shown in Fig. 407, so that at all distances the ball is seen to fall through 

 the same distance at the same rate. The ball is always visible for less 

 than *02 sec., which excludes the influence of movement of the eyes. With 



1 Arch.f. Anat., PhysioL u. wissensch. Med., Leipzig, 1865, S. 152. 



2 Ztschr.f. PsychoL u. PhysioL d. Sinncsorg., Hamburg u, Leipzig, 1892, Bd. iii. S. 21. 



