8o8 



A MANUAL OF PHYSIOLOGY 



visual sensation of a red or green object, the tactile sensation of a 

 smooth and round object, and the gustatory and olfactory sensations 

 which we call the taste or flavour of an apple. And as it is by ex- 

 perience that the child learns to label this bundle of sensations with 

 a spoken, and afterwards with a written name, so it is by experience 



A 



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FIG. 307. ILLUSION OF PARALLEL LINES (HERING) 



that it learns to group the single sensations together, and to make 

 the induction that if the hand be stretched out to a certain distance 

 and in a certain direction (i.e., if various muscular movements, also 



associated with sen- 

 sations, be made), 

 the tactile sensation 

 of grasping a smooth 

 round body will be 

 felt, and that if the 

 further muscular 

 movements involved 

 in conveying it to the 

 mouth be carried out, 

 a sensation agreeable 

 to the youthful palate 

 will follow. At length 

 the child comes to be- 



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FIG. 308. IILUSION OF PARALLEL LINES (ZCLLNER). 



lieve, and, unless he happens to be specially instructed, carries his 

 belief with him to his grave, that when he looks at an apple he sees a 

 round, smooth, tolerably hard body, of definite size and colour; 



while in reality all 

 A B C that tne sense O f 



sight can inform 

 him of is the dif- 

 ference in the in- 

 tensity and colour 

 of the light falling 

 D on ms retina when 



^ ^ > <C he turns his head 



FIG. 309. ILLUSIONS OF SPACE-PERCEPTION. direction 



An interesting illustration of the role of experience in shaping our 

 visual judgments is found in the sensations of persons born blind and 

 relieved in after-life by operation. A boy between thirteen and four- 

 teen years of age, operated on by Cheselden, thought all the objects 

 he looked at touched his eyes. He forgot which was the dog and 



