316 PERSONAL PREPARATION FOR RESEARCH. 



an object, and although the rays of light proceeding from 

 it strike our eye and excite the optic nerve, unless our 

 attention is directed to it we do not perceive it. 



Observation differs from experiment. By experiment we 

 evolve facts, by observation we find them. Neither experi- 

 ments nor other phenomena of nature impart to us new 

 knowledge unless we observe their effects, because it is in 

 the very act of observing that knowledge is acquired by 

 the mind. In observing, we simply notice and record the 

 conditions and phenomena presented to us, and it matters 

 not whether they are those of prepared experiments, or 

 those of phenomena over which we have no control. 



Qualitative observation is very important. The dis- 

 covery of a fact, however strongly it may have been anti- 

 cipated or predicted, is not completely or really made until 

 it has been actually observed. Accurate quantitative 

 observation is also important ; Tycho Brahe made an 

 immense number of most exact records of the positions of 

 the heavenly bodies with the aid of the best of astrono- 

 mical instruments, and these records afterwards became 

 the foundation of Kepler's well-known laws, and of modern 

 astronomy. 



Observation is closely connected with manipulation in 

 making an experiment. Not only must the investigator 

 manipulate so as to produce the desired effect, but he must 

 know how to observe it when it does occur, also know what 

 to expect, where to look for it, and what precautions to 

 take in order to best perceive it. In some cases, before we 

 actually make an experiment we write out a list of all the 

 effects that we can imagine to be likely to appear, and 

 this is a very good plan, because it points out to us what 

 to look for. A logical habit of thought helps us in form- 

 ing such a list, because it aids us in defining clearly the 

 greatest possible number of alternatives. 



