192 UNKNOWN SENSES. 



how different the world may^I was going to say must 

 — appear to other animals from what it does to us. 

 Sound is the sensation produced on us when the vibra- 

 tions of the air strike on the drum of our ear. When 

 they are few, the souud is deep ; as they increase in 

 number, it becomes shriller and shriller; but when they 

 reach 40,000 in a second, they cease to be audible. 

 Light is the effect produced on us when waves of light 

 strike on the eye. When 400 millions of millions of 

 vibrations of ether strike the retina in a second, they 

 produce red, and as the number increases the color 

 passes into orange, then yellow, green, blue, and violet. 

 But between 40,000 vibrations in a second and 400 

 millions of millions we have no organ of sense capable 

 of receiving the impression. Yet between these limits 

 any number of sensations may exist. We have five 

 senses, and sometimes fancy that no others are possible. 

 But it is obvious that we cannot measure the infinite 

 by our own narrow limitations. 



Moreover, looking at the question from the other 

 side, we find in animals complex organs of sense, richly 

 supplied with nerves, but the function of which we are 

 as yet powerless to explain. There may be fifty other 

 senses as different from ours as sound is from sight ; 

 and even within the boundaries of our own senses there 

 may be endless sounds which we cannot hear, and 

 colors, as different as red from green, of which we have 

 no conception. These and a thousand other questions 

 remain for solution. The familiar world which sur- 

 rounds us may be a totally different place to other 

 animals. To them it may be full of music which we 

 cannot hear, of color which we cannot see,* of sensations 

 which we cannot conceive. To place stuffed birds and 



