236 Mr. G. F. Fitzgerald. Electromagnetic Theory [Jan. 9, 



different spectra, inasmuch as their absorption-bands vary in position 

 and in intensity. 



(3.) The photographic absorption spectra can be employed as a 

 means of identifying organic substances, and as a most delicate test 

 of their purity. The curves obtained by co-ordinating the extent of 

 dilution with the position of the rays of the spectrum absorbed by 

 the solution form a strongly marked and often a highly characteristic 

 feature of many organic compounds. 



There is a curious feature in connexion with the position of the 

 absorption bands ; at the less refrangible end they either begin at line 

 12 Cd or line 17 Cd, and those which begin at 12 end a little beyond 

 17. 



No naphthalene or anthracene derivatives have yet been examined, 

 and very few substances of unknown constitution — hence most in- 

 teresting results may be anticipated from a continuation of this 

 research, and this contribution must be accepted rather as a bare com- 

 mencement of the subject than its conclusion. 



II. " On the Electromagnetic Theory of the Reflection and Re- 

 fraction of Light." By George Francis Fitzgerald, 

 M.A., Fellow of Trinity College, Dublin, Communicated 

 by G. J. Stoney, M.A., F.R.S., Secretary of the Queen's 

 University, Ireland. Received October 26, 1878. 



(Abstract.) 



The media, at whose surfaces reflection and refraction are supposed ., 

 to take place, are assumed to be non-conductors, and isotropic as 

 regards magnetic inductive capacity. Some reasons are advanced 

 why the results should apply at least approximately to conductors. 

 In the first part of the paper the media are not assumed to be isotropic 

 as regards electrostatic inductive capacity, so that the results are 

 generally applicable to reflection and refraction at the surfaces of 

 crystals. I use the expressions given by Professor J. Clerk Maxwell 

 in his " Electricity and Magnetism," vol. ii, Part IV, chap. 11, for the 

 electrostatic and electrokinetic energy of such media. By assuming 



three quantities, £, jy, and such that, t representing time, 



Cut Cut 



and are the components of the magnetic force at any point, I have 



thrown these expressions for the electrostatic and electrokinetic energy 

 of a medium into the same forms as M'Cullagh assumed to represent 

 the potential and kinetic energy of the ether, in " An Essay towards a 

 Dynamical Theory of Crystalline Reflection and Refraction," pub- 



