EXPERIMENT. 25 



binations; but this being impracticable, the experimentalist 

 necessarily abandons strict logical method, and trusts to 

 his own insight. Analogy, as we shall afterwards see, 

 gives some assistance, and attention will probably be con- 

 centrated on those kinds of conditions which have been 

 found important in like cases. But we are now entirely 

 in the region of probability, and the experimenter, while 

 he is confidently pursuing what he thinks the right clue, 

 may be entirely overlooking the one condition whose im- 

 portance has been hitherto unsuspected. It is an impres- 

 sive lesson, for instance, that Newton pursued all his 

 exquisite researches on the spectrum unsuspicious of the 

 fact that if he reduced the hole in the shutter to a narrow 

 slit, all the mysteries of the bright and dark lines were 

 within his grasp, provided of course that his prisms were 

 sufficiently good to define the rays. In a similar manner 

 we know not what slight alteration in the most familiar 

 experiments may not open the way to realms of new 

 discovery. 



Many additional practical difficulties encumber the pro- 

 gress of the physicist. It is often impossible to alter one 

 condition without altering others at the same time ; and 

 thus we may not get the pure effect of the condition in 

 question. Some conditions may be absolutely incapable 

 of alteration ; others may be with great difficulty, or only 

 in a certain degree, removable. A very treacherous source 

 of error is the existence of unsuspected conditions, which 

 we of course cannot remove except by accident. These 

 difficulties we will shortly consider in succession. 



It is often beautiful to observe how the alteration of a 

 single circumstance conclusively explains a phenomenon. 

 An excellent instance is found in Faraday's investigation 

 of the behaviour of Lycopodium spores scattered on a 

 vibrating plate. It was observed that these minute spores 

 collected together at the points of greatest motion, whereas 



