400 THE PRINCIPLES OF SCIENCE. [CHAP. 



double refracting substance ; and Sir John Herscliel'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 unaccountable 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 organised 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 sources 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 

 muscular powers, and thus produce unusual combinations 

 and conditions of phenomena, we are said to experiment. 

 Herschel justly remarked l that we might properly call 

 these two modes of experience passive and active observa- 

 tion. In both cases we must certainly employ our senses 

 to observe, and an experiment differs from a mere observa- 

 tion 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 

 experiment. When the earliest astronomers simply noticed 

 the ordinary motions of the sun, moon, and planets upon 

 the face of the starry heavens, they were pure observers. 

 But astronomers now select precise times and places for 

 important observations of stellar parallax, or the transits 

 of planets. They make the earth's orbit the basis of a 

 well arranged natural experiment, as it were, and take well 

 considered advantage of motions which they cannot 

 control. Meteorology might seem to be a science of pure 



1 Preliminary Discourse on the Study of Natural Philosophy, p. 77. 



