SPATIAL PERCEPTION. 1139 



lated by the same point ; and Schon supposes that this difference 

 of intensity, assisted by differences of quality due to differences of 

 sensibility to colour, acts as the basis of the ideas of distance. Schon, 

 in fact, suggests that differences of quality act as local signs in the 

 retina, in much the same way that Lotze supposes them to act in the 

 skin. The objections to this theory are, first, that it is doubtful 

 whether the differences between the nasal and temporal halves described 

 by Schon exist. Other observers have found slight differences, but 

 very much less than those described by Schon ; secondly, the sensibility 

 of the retina is continually varying with the condition of adaptation, and 

 varying adaptation should produce variations in our estimation of distance, 

 which is not found to be the case ; and lastly, this explanation, as Schon 

 himself acknowledges, only applies to indirect vision, for images of 

 objects in the same line as the fixation point will fall either on the 

 nasal halves or on the temporal halves, which have, according to Schon, 

 equal sensitiveness. 



To explain this latter kind of localisation, Schon suggests another 

 explanation, another kind of local sign, which he calls the "rivalry 

 sign." Since images of an object beyond the fixation point fall on the 

 more sensitive nasal halves, and those of objects nearer the fixation 

 point on the less sensitive temporal halves, the former will more easily 

 prevail in the rivalry ; and Schon supposes that this difference acts as 

 the basis of estimation in the case of direct vision. 



Perception of form.— The physiological basis of localisation in 

 space has now been considered at some length, and especially of localisa- 

 tion in depth. This localisation in space is the chief factor in various 

 kinds of more complex perception, and these, namely, the perception 

 of form and size, can now be more briefly considered. 



Certain very simple cases of perception of form were dealt with in 

 the second part of this article, and in such cases the process seems to 

 be comparatively simple. In the case of very small objects, such as 

 letters, it is doubtful how far the form of the retinal image is the sole 

 physiological factor, and how far perception depends on minute move- 

 ments of the eyes following the outline of the object. In the case of 

 large objects, movements must be of greater influence. The pheno- 

 menon of metamorphopsia is evidence of the view that the retinal 

 factor is very important. This occurs in retinitis, especially in the 

 early stages ; the forms of simple objects are found to be distorted, a 

 straight line appearing curved or irregular. Its occurrence in the early 

 stages of retinitis points to its dependence on inflammatory exudation, 

 disturbing the relative position of retinal elements, so that a row of such 

 elements which have previously been in a straight line becomes distorted. 

 A linear retinal image will therefore fall on retinal elements which 

 have normally occupied a curved or irregular position, and have been 

 previously stimulated by curved or irregular objects, and a straight line 

 will consequently appear curved or irregular. 



The perception of form in three dimensions, i.e. of solidity, depends 

 primarily on the physiological binocular mechanism for relative distance, 

 bat is very greatly influenced by psychological factors, and especially by 

 the distribution of light and shade, as is shown by Wheatstone's 

 pseudoscope and by other experiments in which reversal of the apparent 

 direction of incidence of light produces illusions of relief. 



Perception of size. — The two chief factors on which depends the 



