222 PRINCIPLES OF PHYSIOLOGY 



animals may hear sounds that are inaudible 

 to man. Our knowledge, therefore, of the 

 external world is limited by our senses, and 

 there may be many phenomena for which 

 we have no powers of perception. For 

 example, we have no organ for the perception 

 of changes in the electrical condition of sur- 

 rounding matter, and were we supplied with 

 such an organ a new world would be opened 

 up. 



131. The delicacy of the sense organs is 

 remarkable. Thus we may detect a pressure 

 on the skin of -002 gram. We can detect the 

 eighth of a degree centigrade when the 

 temperature of the skin is 18 C. The 

 shortening of a muscle may be detected so 

 small as -004 of a millimetre (l-6,000th of an 

 inch). The ear can detect vibrations of sound 

 caused by movements of molecules of the air 

 of -0004 mm. (the 1-600, 000th of an inch, or 

 1-1 Oth of the wave length of green light ; 

 while the retina is even more sensitive ; the 

 energy of the feeblest light that can be dis- 

 tinguished at a certain distance, say 100 

 yards, is of the same order of magnitude as 



