962 JUDGMENT OF SIZE. [BOOK in. 



On the other hand the visual perception or visual judgment 

 is not formed exclusively out of the visual sensations which are 

 excited by the image of an object falling on one or on both eyes 

 in a given position. In looking at an object, a movement of one 

 or both eyes often takes place, and the perception of the object or 

 a judgment concerning the object is formed out of the two (or 

 more) sensations excited by the same object in different posi- 

 tions of the eyes. And here other factors enter into the pro- 

 cess, namely sensations other than visual sensations, sensations 

 connected with the contractions of the muscles of the eye, 

 affections of what is known as " the muscular sense." These 

 come into play even when we use one eye only, but are espe- 

 cially potent when we use both eyes in binocular vision ; a large 

 number of our visual judgments are determined by the muscular 

 sensations derived from the movements of the eyes through 

 which we look at the object whose features we are judging. 



Other influences also, such for instance as sensations of 

 touch, take part in the psychical processes in question. The 

 mere visual sensations which external objects excite, the imme- 

 diate and direct effects of the visual impulses, form after all but 

 a small part of what we call our vision. Such sensations and 

 other like sensations derived through other senses are to us but 

 symbols of things, upon which the mind puts its own interpre- 

 tation. But into these matters we cannot enter here. We must 

 confine ourselves to certain common facts concerning perceptions, 

 illusions and visual judgments, and more especially to those which 

 relate to the size and distance of external objects and to the 

 characters of form which are indicated by the word "solidity." 



599. Appreciation of Apparent Size. The foundation of 

 our judgment of the size of any object is the size of the retinaj. 

 image of the object. We can distinguish a sensation involving 

 a large retinal area from one involving a small area, and in the 

 region of distinct vision can appreciate even small differences ; 

 this is of course only an exercise of the power of localization. 

 We have seen however that, even in the case of a simple and 

 single sensation such as that of a white patch on a black 

 ground, the sensation does not correspond exactly to the ob- 

 jective stimulation of the retina; the white patch through 

 irradiation 582 appears larger than it really is. When we 

 come to deal with more complex groups of sensations we find 

 that over and above any such physiological modifications of the 

 sensations, the psychical processes mentioned above affect our 

 perceptions and judgments of size, often giving rise to illusions. 



If a line such as AC, Fig. 

 162, be divided into two 



-p m 162 equal parts AB,BC, and AB 



be divided by distinct marks 



into several parts, as is shewn in the figure, while BC\>Q left 



