246 EECENT PROGEESS OF THE THEOSY OF VISION. 



that colours, when perceived at the same time and place, 

 melt into compound colours. The eye cannot tell the 

 difference, if we substitute orange for red and yellow ; but 

 if we hear the notes C and E sounded at the same time, 

 we cannot put D instead of them, without entirely 

 changing the impression upon the ear. The most com- 

 plicated harmony of a full orchestra becomes changed to 

 our perception if we alter any one of its notes. No accord 

 (or consonance of several tones) is, at least for the practised 

 ear, completely like another, composed of different tones ; 

 whereas, if the ear perceived musical tones as the eye 

 colours, every accord might be completely represented by 

 combining only three constant notes, one very low, one 

 very high, and one intermediate, simply changing the 

 relative strength of these three primary notes to produce 

 all possible musical effects. 



In reality we find that an accord only remains un- 

 changed to the ear, when the strength of each separate 

 tone which it contains remains unchanged. Accordingly, if 

 we wish to describe it exactly and completely, the strength 

 of each of its component tones must be exactly stated. 



In the same way, the physical nature of a particular 

 kind of light can only be fully ascertained by measuring 

 and noting the amount of light of each of the simple 

 colours which it contains. But in sunlight, in the light 

 of most of the stars, and in flames, we find a continuous 

 transition of colours into one another through numberless 

 intermediate gradations. Accordingly, we must ascertain 

 the amount of light of an infinite number of compound 

 rays if we would arrive at an exact physical knowledge of 

 sun or starlight. In the sensations of the eye we need 

 distinguish for this purpose only the varying intensities 

 of three components. 



The practised musician is able to catch the separate 

 note? of the various instruments among the complicated 





