THE PRINCIPLES OF SCIENCE. 



vertebra ; Mains unintentionally examined with a double 

 refracting substance light reflected from distant windows ; 

 and Sir John Herschel s attention was drawn to the 

 peculiar appearance of a solution of quinine sulphate. In 

 earlier times there must have been some one who first 

 noticed the strange behaviour of a loadstone, or the un 

 accountable motions produced by amber. As a general 

 rule we shall not know in what direction to look for a 

 great body of phenomena widely different from those 

 familiar to us. Chance then must give us the starting 

 point ; but one accidental observation well used may lead 

 us to make thousands of observations in an intentional and 

 organized manner, and thus a science may be gradually 

 worked out from the smallest opening. 



Distinction of Observation and Experiment. 



It is usual to say that the two modes of experience are 

 Observation and Experiment. When we merely note and 

 record the phenomena which occur around us in the 

 ordinary course of nature we are said to observe. When 

 we change the course of nature by the intervention of our 

 will and muscular powers, and thus produce unusual com 

 binations and conditions of phenomena, we are said to 

 experiment. Sir John Herschel has justly remarked b that 

 we might properly call these two modes of experience 

 passive and active observation. In both cases we must 

 certainly employ our senses to observe, and an experiment 

 differs from a mere observation in the fact that we more 

 or less influence the character of the events which we 

 observe. Experiment is thus observation plus alteration 

 of conditions. 



It may readily be seen that we pass upwards by in 

 sensible gradations from pure observation to determinate 



b Preliminary Discourse on the Study of Natural Philosophy/ p. 77. 



