THE SIZE OF A TOMS. 195 



proper appliances to the dynamical models before 

 you to account for refractive dispersion in an 

 infinitely fine-grained structure. It is not seven- 

 teen hours since I saw the possibility of this 

 explanation. I think I now see it perfectly, but 

 you will excuse my not going into the theory 

 more fully under the circumstances. 1 The difficulty 

 of Cauchy's theory has weighed heavily upon me 

 when thinking of bringing this subject before 



- 



you. I could not bring it before you and say 

 there are only four particles in the wave-length, 

 and I could not bring it before you without saying 

 there is some other explanation. I believe another 

 explanation is distinctly to be had in the manner 

 I have slightly indicated. 



Now look at those beautiful distributions of 

 colour on the screen before you. They are dif- 

 fraction spectrums from a piece of glass ruled 

 with 2000 lines to the inch. And again look, and 



1 Further examination has seemed to me to confirm this first im- 

 pression ; and in a paper " On the Dynamical Theory of Dispersion," 

 read before the Royal Society of Edinburgh on the 5th of March, 

 I have given a mathematical investigation of the subject. W. T., 

 March 16, 1883. 



O 2 



