n 3 6 VISION. 



the nearer of two objects may be localised in the line passing from the 

 farther object to the root of the nose, and not in the line passing to the 

 open eye. This reference to the cyclopean eye is not constant ; and by 

 practice in using one eye, as in shooting, it is probable that localisation 

 in space may take place in reference to the visual direction of that eye. 



In connection with the idea of the cyclopean eye, it is interesting 

 that we are unable to tell with which eye we are seeing an object. If a 

 piece of black pasteboard with a pinhole in the middle is moved about 

 till the hole is in front of one eye, it may appear to be midway between 

 the two eyes, and mistakes are frequently made in naming the eye which 

 is exposed to the light (Kogers). 1 An after-image obtained by exposure 

 of one eye may appear to be seen with the other eye. The familiar 

 experiment in which a tube on the right side of an opaque object 

 seems, when the right eye is applied to the tube, to penetrate the 

 object, is another example. On the other hand, Helmholtz 2 noticed that 

 if one of two stereoscopic photographs had on it a spot, it gave rise to an 

 appearance of dimness in the combined picture, and that he found 

 himself involuntarily rubbing the eye of the same side as the spot. 

 Helmholtz supposed that this indicated that he was sensible of the eye 

 stimulated. 



Psychological factors in localisation. — These are often known as 

 the empirical factors, but since this name seems to assume that the 

 other factors are not based on experience, it is perhaps better to avoid 

 this nomenclature. 



The psychological factors play a very important part in localisation 

 of the fixation point, but are also of great importance in localisation 

 relative to the fixation point. They are the only factors in question 

 for localisation beyond a certain distance from the eyes, namely, that 

 distance at which the binocular retinal mechanism ceases to be of 

 value, probably about 20 metres. The appearance of distance produced 

 in paintings is due exclusively to the application of devices based on 

 these psychological factors, and it is on this account that the appearance 

 of distance in pictures, as was pointed out by Francis Bacon, is more 

 marked when only one eye is used. Our ideas of distance and size are 

 very closely related ; it is probable that in general the idea of distance 

 is primary, and that our idea of the size of an object depends on the 

 estimated distance of the object (p. 1 140), but, when the size is known, 

 it may greatly influence depth perception. This is shown in a striking 

 way in Hering's fall experiment. Observers tend to localise the larger 

 balls nearer than the fixation point, the smaller farther than the fixation 

 point, and they may be unconscious of the factor upon which their 

 erroneous localisation has depended. Linear and aerial perspective 

 have great influence on the apparent distance of an object, while the 

 distribution of light and shade is especially important in producing the 

 appearance of relief and solidity. The parallactic movements of objects 

 at different distances on moving the head may also be important factors 

 in depth perception. 



Theories of localisation. — Most of the older theories ignored the 

 distinction between relative and absolute localisation. At one time it 

 was generally accepted that the estimation of distance depended on 

 movement; one object was supposed to appear nearer than another, 



1 Am. Journ. Sc, New Haven, 1860, vol. xxx. p. 404. 



2 " Handbuch d. physiol. Optik," 1867, S. 744. 



