4 2o SCIENCE PROGRESS. 



will be the nearness of the frequency of vibration of the 

 atom to the frequencies of vibration of the waves we are 

 studying. If the atom frequency is only a little greater 

 than that of the waves its effect upon them will be great, 

 and the velocity of propagation of the waves will be slower 

 than if the atom vibration were of much greater frequency 

 than that of the waves. 



That atoms of matter can vibrate at frequencies some- 

 what greater than those concerned in the light we use when 

 studying dispersion is manifest from the spectra they 

 exhibit. The very great- opacity of all known transparent 

 substances, even of gases, to ultra-violet light of frequencies 

 of vibration not very much greater than the frequencies 

 usually studied proves that all these bodies are capable 

 of absorbing vibrations of the frequency of these ultra-violet 

 vibrations, and are, consequently, capable of themselves 

 vibrating at these frequencies. There is, consequently, no 

 objection such as could be urged against the other theory 

 of dispersion to this, what may be called, resonant theory 

 of dispersion. In one case the two theories run into one 

 another to some extent. When masses are regularly dis- 

 tributed through a medium, we may consider the medium 

 between any pair of masses as capable of independent 

 vibration : it could, in general, vibrate with a node at each 

 mass and a set of intervening loops. The slowest of these 

 rates of vibration would be comparable with that of a wave 

 whose length was double the distance between the masses 

 so that we might consider the phenomenon as due either to 

 a comparability of the wave lengths to the distances be- 

 tween the masses, or to a comparability of the frequencies 

 to the frequencies of vibration of the medium between 

 the masses. The subject does not seem of much immediate 

 interest, however, as phenomena depending on these very 

 short waves have not been sufficiently studied to be capable 

 of discussion in connection with a theory of this kind. 



It is evident, then, that anything which alters the vibra- 

 tion frequencies of the atoms of matter must alter dispersion. 

 Altering the density of matter will also, of course, alter the 

 rate of propagation of light : it will alter the grip that 



