38 AN INTRODUCTION TO SCIENCE 



instant upon the retina produce but one mean effect, which 

 might have been produced by a single colour. The eye 

 has no power of analysing super-imposed vibrations of light. 

 A harmony of several colours is to the eye what a melody 

 is to the ear. But the eye is for the recognition of position 

 in space; the harmony of colours must be stationary. A 

 sequence of colours is not only not enjoyable, but actually 

 painful. The ear, on the contrary, reports sequence in 

 time, and has hardly anything to do with position in space. 



Of taste and of "common sensation" we need say but 

 little. The former has so personal an application in decid- 

 ing what we shall swallow that it can hardly be said to give 

 us any information as to the properties of the things which 

 belong to the external world ; the information reaches our 

 brain only at the moment when these things are passing into 

 our inside selves ; while the latter in its several varieties of 

 sense of touch, of temperature, of pressure, and of muscular 

 exertion, gives us information chiefly about our outside 

 selves. But concerning the sense of touch used in conjunc- 

 tion with the sense of sight much might be said ; for it is to 

 this cooperation that we owe all that we know as to the 

 shape and position of the objects by which we are sur- 

 rounded. To take a simple illustration : A flash of light- 

 ning illuminates the interior of a darkened room. We see 

 it as a space bounded by walls and occupied by various 

 solid objects ; for thus we interpret the image formed on the 

 two retinae of our eyes. But if this illuminated room were 

 the first thing seen by a blind man it would convey no mean- 

 ing to his mind. His sense of touch would have told him 

 that the room is bounded by walls and that it contains solid 

 objects. But he would be unable without training to cor- 

 relate what he had felt with what he now saw. His eyes, 

 used now for the first time, show him a flat picture ; they 

 give him no information regarding the third dimension. 

 Suppose that in this room there is a round ball resting upon 

 the table. The man's right and left eyes each show him a 



