THE SENSES 



129 



the direction of the rays, we can be conscious only of 

 different changes in different parts of the visual centre depen- 

 dent on the changes set up in the retinae. We have there- 

 fore only the means of getting a flat picture of what we look 

 at, but no special arrangement for having different sensations 

 according to the distance of an object or according to whether 

 it is flat or in relief. Thus the means of determining the 

 form and size of objects by the retinas and visual centres is 

 very limited. 



To gain knowledge of the size, distance, or contour of an 

 object we have to combine certain other sensations, or certain 

 past experiences with the visual sensation, and to form a 

 judgment or concept of what we are looking at. 



It is not then wonderful that erroneous judgments are 

 frequently made. The possibility of such may be indicated 

 by one or two examples of what are called modified per- 

 ceptions. 



(1) If an imaginary line be divided into two equal parts, 

 and if a series of dots are put 



along the one half, that part will 



appear longer than the other. 



(2) If a black wafer on a white ground, and a white wafer 

 on a black ground be looked at, the latter will appear larger 

 than the former. 



(3) If a square be ruled with parallel diagonal lines, and 

 if short vertical and horizontal lines be placed alternately 

 upon them, they will no longer appear 



parallel. 



IY. Hearing. 



While through the sense of touch 

 we are made aware of differences of 

 pressure, through the sense of hearing 

 certain vibratory changes of pressure 

 specially affect the consciousness. Even 

 simple organisms, devoid of any special 

 organ of hearing, may be affected by vibratory changes, and 

 in fish it is difficult to be certain how far such vibrations 

 produce their effect through the ear or through the body 

 generally; but in higher vertebrates it is chiefly through 



9 



FIG. 67. To illustrate 

 Optical Illusions. 



