120 REPORT— 1882. 



Report of the Covimittee, consisting of Professor Dewar, Dr. 

 Williamson, Dr. Marshall Watts, Captain Abney, Mr. Stoney, 

 Professor Hartley, Professor McLeod, Professor Carey Foster, 

 Professor A. K. Huntington, Professor Emerson Reynolds, Pro- 

 fessor Reinold, Professor Liveing, Lord Eayleigh, Dr. Schuster, 

 and Professor W. Chandler Roberts {Secretary), appointed for 

 the purpose of reporting tipon the present state of out Know- 

 ledge of Spectrum Analysis. 



The Genesis of Spectra. By Dr. Schuster. 



It is the ambitious object of Spectroscopy to study the vibrations of 

 atoms and molecules in order to obtain what information we can on the 

 nature of the forces which bind them together. The vibrations we know 

 must be of a very complicated nature, yet it is natural that not many 

 years after Spectrum Analysis was raised to the rank of a science by the 

 labours of Kirchhoff and Bunsen attempts were made to discover a law 

 in the apparent irregularity with which different lines of the same element 

 are distributed over the spectrum. If an atom can vibrate in more ways 

 than one, it is certain that some connection must exist between the 

 different periods, and this connection we may attempt to find out by trial. 

 Or we may speculate on the causes which produce such vast differences 

 in the chemical properties of some of the elements, while other elements 

 have properties which resemble each other to an equally marked degree. 

 We may be led on by such speculations to try whether we can trace any 

 similarity in the periods of vibration of molecules which have similar 

 chemical properties, or we may endeavour to classify the elements accord- 

 ing to their spectra, and see whether such a classification would divide 

 the elements into groups agreeing with those into which they have been 

 divided by means of their chemical and physical behaviour. 



When different elements combine together the vibrations of the com- 

 pound molecule are not obtained by the simple addition of the periods of 

 the elements. The spectrum of a molecule is entirely distinct from that of 

 its elements, and we may well ask the question whether we can trace in 

 the spectrum of the compound the influence of the different atoms com- 

 posing it. Thus, for instance, we might trace some relationship between 

 the spectra of the oxides, bromides, chlorides, or iodides of a metal and 

 that of the metal itself, or we may in the absorption spectrum of a salt 

 trace one part to the influence of the base, the other to the influence of 

 the acid. Such and similar questions have been raised and have been 

 partially answered. But we must not too soon expect the discovery of 

 any grand and very general law, for the constitution of what we call 

 a molecule is no doubt a very complicated one, and the difficulty of the 

 problem is so great that were it not for the primary importance of the 

 result which we may finally hope to obtain, all but the most sanguine 

 might well be discouraged to engage in an inquiry which, even after many 

 years of work, may turn out to have been fruitless. We know a great 

 deal more about the forces which produce the vibrations of sound than 

 about those which produce the vibrations of light. To find out the 

 different tunes sent out by a vibrating system is a problem which may or 

 may not be solvable in certain special cases, but it would baffle the most 



