Model to illustrate Helmholtz's Tlieory of Dispersion, 56 1 



Method of Observation. 



The model was driven by means of pendulums, and also by 

 an ordinary metronome. The latter is on the whole more 

 convenient ; the frequencies must, of course, be determined 

 by timing and not from the graduations of the instrument. 



The motion becomes regular very soon after the metronome 

 is started. Standing waves are set up which enable the 

 velocity to be readily found. 









Fig. 4. 

















1 













H 



1 







J=- nat. size. 



35 



Results of the Experiments. 



The results obtained are given in fig. 2. 



The ordinates are the reciprocals of the velocities, and 

 hence proportional to the refractive index. All the velocities 

 determined are inserted. 



