158 JUDGMENT OF SIZE. [BOOK in. 



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 positions 

 of the eyes. And here other factors enter into the process, 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 especially potent when we use 

 both eyes in binocular vision ; a large number of our visual judg- 

 ments 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 immediate 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 interpretation. 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." 



797. Appreciation of Apparent Size, The foundation of 

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

 retinal image of the object. We can distinguish a sensa- 

 tion 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 objective stimulation of the retina ; the white patch through 

 irradiation 780 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. 

 158, be divided into two 



equal parts AB 9 BC, and AB A FJG ^ g 



be divided by distinct marks 



into several parts, as is shewn in the figure, while BC be left 



entire, the distance AB will always appear greater than CB. 



