424 THE PRINCIPLES OF SCIENCE. [CHAP. 



perfectly under control, and its influence can be brought 

 into action, reversed, or stopped by merely touching a 

 button. Thus Faraday was enabled to prove the rotation 

 of the plane of circularly polarised light by the fact that 

 certain light ceased to be visible when the electric current 

 of the magnet was cut off, and re-appeared when the 

 current was made. " These phenomena," he says, " could 

 be reversed at pleasure, and at any instant of time, and 

 upon any occasion, showing a perfect dependence of cause 

 and effect." l 



It was Newton's omission to obtain the solar spectrum 

 under the simplest conditions which prevented him from 

 discovering the dark lines. Using a broad beam of light 

 which had passed through a round hole or a triangular 

 slit, he obtained a brilliant spectrum, but one in which 

 many different coloured rays overlapped each other. In 

 the recent history of the science of the spectrum, one 

 main difficulty has consisted in the mixture of the lines of 

 several different substances, which are usually to be found 

 in the light of any flame or spark. It is seldom possible 

 to obtain the light of any element in a perfectly simple 

 manner. Angstrom greatly advanced this branch of science 

 by examining the light of the electric spark when formed 

 between poles of various metals, and in the presence of 

 various gases. By varying the pole alone, or the gaseous 

 medium alone, he was able to discriminate correctly be- 

 tween the lines due to the metal and those due to the 

 surrounding gas. 2 



Failure in the Simplification of Experiments. 



In some cases it seems to be impossible to carry out the 

 rule of varying one circumstance at a time. When we 

 attempt to obtain two instances or two forms of experi- 

 ment in which a single circumstance shall be present in 

 one case and absent in another, it may be found that this 

 single circumstance entails others. Benjamin Franklin's 

 experiment concerning the comparative absorbing powers 

 of different colours is well known. " I took," he says, " a 



1 Experimental Researches in Electricity, vol. iii. p. 4. 



2 Philosophical Magazine, 4th Series, vol. ix. p. 327. 



