

THE SIZE OF ATOMS. 155 



spectrum. The lowest, marked by Fraunhofer 

 with the letter A, has for wave-length 7i/ioo,oooths 

 of a centimetre. On the model before you I will 

 now show you what is meant by a " wave-length ; " 

 it is not length along the crest, such as we some- 

 times see well marked in a wave of the sea break- 

 ing on a long straight beach ; it is distance from 

 crest to crest of the waves. [This was illustrated 

 by a large number of horizontal rods of wood 

 connected together and suspended bifilarly by 

 two threads in the centre hanging from the ceil- 

 ing ; 1 on moving the lowermost rod, a wave was 

 propagated up the series.] Imagine the ends of 

 those rods to represent particles. The rods them- 

 selves let us suppose to be invisible, and merely 

 their ends visible, to represent the particles acting 

 upon one another mutually with elastic force, as 

 if of indiarubber bands, or steel spiral springs, 

 or jelly, or elastic material of some kind. They 

 do act on one another in this model through the 



1 The details of this bifilar suspension need not be minutely 

 described, as the new form, with a single steel pianoforte wire to 

 give the required mutual forces, described below and represented in 

 Fig. 34, is better and is more easily made. 



