356 FRAGMENTS OF SCIENCE. 



This complex mass of action, emotional, intellectual, 

 and mechanical, is evoked by the impact upon the retina 

 of the infinitesimal waves of light coming from a few 

 pencil marks on a bit of paper. We have, as Lange 

 says, terror, hope, sensation, calculation, possible ruin, 

 and victory compressed into a moment. What caused 

 the merchant to spring out of his chair ? The contrac- 

 tion of his muscles. What made his muscles contract ? 

 An impulse of the nerves, which lifted the proper latch, 

 and liberated the muscular power. Whence this im- 

 pulse ? From the centre of the nervous system. But 

 how did it originate there ? This is the critical ques- 

 ^tion, to which some will reply that it had its origin in 

 the human soul. 



The aim and effort of science is to explain the un- 

 known in terms of the known. Explanation, therefore, 

 is conditioned by knowledge. You have probably heard 

 the story of the German peasant, who, in early railway 

 days, was taken to see the performance of a locomotive. 

 He had never known carriages to be moved except by 

 animal power. Every explanation outside of this con- 

 ception lay beyond his experience, and could not be 

 invoked. After long reflection therefore, and seeing 

 no possible escape from the conclusion, he exclaimed 

 confidently to his companion, f Es miissen doch Pferde 

 darin sein ' There must be horses inside. Amusing as 

 this locomotive theory may seem, it illustrates a deep- 

 lying truth. 



With reference to our present question, some may 

 be disposed to press upon me such considerations as 

 these : Your motor nerves are so many speaking-tubes, 

 through which messages are sent from the man to the 

 world ; and your sensor nerves are so many conduits 

 through which the whispers of the world are sent back 

 to the man. But you have not told us where is the 



