1 1 34 VISION. 



this apparatus Greeff found with monocular vision that the right and 

 wrong answers were approximately equal, while with both eyes the wrong 

 answers formed a very small percentage of the total, varying from 2 to 6 

 per cent., according to the distance of the fixation point, F, the width of 

 the slit, cc, and the distance between the falling ball and the fixation point. 

 He found the experiment succeeded at any distance of the fixation point 

 up to 20 metres, at which distance the falling ball had to be not less than 

 35 cm. from the fixation point. Taking the just perceptible difference 

 at -g 1 ^-, Greeff found that this corresponded to a retinal distance of '002 to 

 003 mm. Diminished visual acuity diminished the influence of binocular 

 vision. At one time Greeff believed that no cases of squint gave good 

 results with the experiment, but later 1 has found that cases do occur in 

 which it seems that the capacity for performing the fall experiment 

 can be acquired after tenotomy. It seems also that such cases may 

 be able to give correct answers with the fall experiment, though unable 

 to combine stereoscopically (Simon). 2 



The fall experiment shows conclusively that there is a binocular 

 mechanism of some kind for the perception of relative distance. It also 

 shows that this mechanism is independent of movements of the eyes, the 

 time during which the falling ball is exposed being too short for a move- 

 ment either of the eyeballs or of accommodation to be completed. That 

 perception of depth is independent of movement, is also shown in other 

 ways. Dove 3 found that the appearance of distance in stereoscopic com- 

 bination occurred with instantaneous illumination. This was doubted by 

 Donders, 4 but on careful investigation he found that, with a suitable 

 position of the eyes, the stereoscopic effect might occur with a single 

 electric spark. Usually more than one was necessary ; the first spark 

 probably only enabled the observer to fix suitably. 



As a proof that binocular relief is independent of movement of the 

 eyes, Wheatstone 6 showed that it may be seen in the after-image. He 

 exposed broad coloured lines on a complementary ground, and found that 

 the binocular after-image showed itself in bold relief in the intervals of 

 the alternation due to rivalry. A similar observation was made by 

 Eogers, 6 who found that the appearance of relief in the after-image also 

 occurred when the lines had been presented to the eyes successively. 



Localisation of fixation point. It is generally supposed that we 

 localise the fixation point in its real position, namely, at the point of 

 intersection of the lines of fixation, and that this localisation depends on 

 our knowledge of the position of the eyeballs, derived either from muscle 

 sensations of peripheral origin, or from " sensations of innervation." There 

 is no doubt, however, that in the absence of retinal factors our knowledge 

 of the position of the eyeballs is very defective. Very large errors may be 

 made in localising a point of light in complete darkness (Exner). 7 In cases 

 of blindness due to optic atrophy, after the eyeballs have been directed 

 to one side, they may slowly return to the median position without 

 the knowledge of the owner, who still believes he is looking to one side 



1 Klin. Monatsbl.f. Augenh., 1895, Bd. xxxiii. S. 352. 



2 Centralbl. f. prakt. Augenh., Leipzig, 1896, Bd. xx. S. 174. 



3 Sitzungsb. d. k. Akad. d. wissensch. zu Berlin, 1841, S. 251. 



4 Arch.f. Ophth., 1867, Bd. xiii. Abtli. 1, S. 29. 



5 Phil. Trans., London, 1838, p. 392. 



H Am. Journ. Sc., New Haven, 1860, vol. xxx. p. 387. 



7 Ztschr. f. Psychol. u. Physiol. d. Sinnesorg., Hamburg u. Leipzig, 1896, Bd. xii. S. 321. 

 Sherrington, on the other hand, found that the eyes can be directed to a given point in com- 

 plete darkness with considerable accuracy (Proc. Roy. Soc. London, 1898, vol. Ixiv. p. 120. 



