viii OCULAR MOVEMENTS 417 



produced by stimulation of the individual sensory elements of 

 the retina. 



The psychical processes entailed in visual perception of the 

 external world are highly complex, whether thi-y are limited to 

 the identification of objects, or, by a more profound mcnt;il 

 elaboration, include judgments as to their characters (size, distance, 

 etc.). This complexity depends on different factors, which are 

 concerned in the transformation of crude sensations into percepts 

 and representations. 



(a) Visual perception depends not only on elementary sensa- 

 tions, but also on the memory of previous sensations and 

 perceptions : suggestion, recollections, and the association of past 

 with present sensations are weighty factors in the formation of 

 visual percepts and judgments. Every percept, therefore, while 

 it adds something to our experience that is, to the sum of our 

 memories, is itself in part the result of them. The identification 

 of an object necessarily presupposes exact knowledge of it, but the 

 perception of an unknown tiling also connotes the remembering 

 of known objects which it more or less resembles, hut from which 

 it differs in some specific character. 



(b*) Visual perception is the result not only of a mental 

 synthesis of the sum total of the countless and manifold 

 elementary sensations excited simultaneously by an object in 

 one or both eyes, but also of those aroused by the rapid alternation 

 and succession of the images due to the movements of the eyes 

 and head, i.e. the variety and multiplicity of the sensations 

 aroused by the same object regarded from different points of 

 view. 



(c) Visual perception is further aided by sensations other 

 than visual, as the muscular sensations, which accompany the 

 movements of convergence, accommodation, and pupillary reactions. 

 The two last come into play in simple uuiocular vision, the first 

 only in binocular vision. 



The mental synthesis by which the complex simultaneous or 

 successive elementary sensations are transformed into perceptions 

 fails, not infrequently, to correspond with the real objects. The 

 erroneous character of many of our perceptions (distinguished by 

 the name of optical illusions') can in a number of cases be readily 

 demonstrated ; but they often persist, even when we are con- 

 vinced of the fallacy. These are interesting, not merely because 

 they are a striking demonstration of the relativity of our visual 

 judgments (and, generally speaking, of the whole of our knowledge 

 acquired through the senses), but also because they enable us to 

 penetrate a little deeper into the analysis of the mental processes 

 which underlie our perceptions. 



Must interesting of all is the investigation of our ability, by 

 means of vision, to appreciate the size, distance, and form of the 



VOL. IV 2 E 



