36 AN INTRODUCTION TO SCIENCE 



colours in varying proportions. If, for example, the rays 

 vibrate at the rate of 580 billions to the second, the eye says 

 that they partake equally of the characters of red and green, 

 with a very small trace of violet, and the brain gives to this 

 combination the quality of yellow. We cannot imagine 

 what the sensation the colour would be like if the eye 

 contained a mechanism specially sensitive to the rays which, 

 when stimulating equally the red mechanism and the green 

 mechanism, are judged to be yellow. 



Hearing. A comparison of the cochlea of the human 

 ear with that of animals shows that Man possesses an organ 

 of hearing which is as elaborate in structure as any to be 

 found in the animal kingdom ; and such observations as are 

 available indicate that he can put it to far better use in the 

 analysis of sound than any animal can. Indeed the most 

 remarkable characteristic of the ear, as an organ for discrim- 

 inating sounds of different wave-lengths, is its almost 

 unlimited capability of improvement under training. An 

 untrained savage cannot discriminate a difference of less 

 than a semitone between two notes, whereas a trained 

 musician detects a discrepancy of one-thirtieth of a semi- 

 tone, or even less. And not only can the ear discriminate 

 minute differences in rate of vibration, but it can in a very 

 remarkable degree resolve compound waves of sound into 

 their constituent waves. No tone which reaches the ear is a 

 pure tone. Upon the vibrations of a certain rapidity which 

 constitute its prime tone are superposed numbers of 

 harmonic vibrations of rapidity greater than that of the 

 prime tone in the proportions of f , f , f , and so on. The ear 

 detects the presence of these overtones and recognises their 

 relative preponderance or the "quality" of the note pro- 

 duced by a musical instrument. As an analytic apparatus 

 the ear is far more efficient than either of the other sense- 

 organs. 



Animals have little need of the power of analysing 

 sounds. To a cat all mice squeak alike, we may presume ; 



