THE SIZE OF ATOMS. 189 



a distance as (in this case equal to i/24th of the 

 wave-length) to bring the crest of the wave to 

 midway between two molecules. This pair of 

 diagrams (Figs. 38 and 39) shows the same for a 

 wave having four molecules in the wave-length, 

 and this pair (Figs. 40 and 41) for a wave having 

 two molecules in the wave-length. 



The more nearly this critical case is approached, 

 that is to say, the shorter the wave-length down 

 to the limit of twice the distance from molecule 

 to molecule, the less becomes the difference 

 between the two configurations of motion con- 

 stituted by waves travelling in opposite directions. 

 In the extreme or critical case the difference is 

 annulled, and the motion is not a wave motion, 

 but a case of what is often called "standing 

 vibration." Before I conclude this evening I hope 

 to explain in detail the kind of motion which we 

 find instead of wave-motion (become mathe- 

 matically imaginary), when the vibrational period 

 of the exciter is anything less than the critical 

 value, because this case is of extreme importance 

 and interest in physical optics, according to 



