REPORTS 



ON 



THE STATE OF SCIENCE. 



Catalogue of the Oscillation-frequencies of Solar Rays ; drawn up under 

 the superintendence of a Committee of the British Association, consist- 

 ing of Dr. Huggins (Chairman), Dr. De La Rue, Mr. J. Norman 

 Lockyer, Dr. J. Emerson Reynolds, Mr. Spottiswoode, Dr. W. 

 Marshall Watts, and Mr. G. Johnstone Stoney (Reporter) *. 



Eveey periodic disturbance of the ether which can be propagated through 

 it as an undulation may be represented mathematically by one or more 

 terms of the harmonic expansion known as Fourier's series. A single term 

 of this scries suffices to represent the undulation when the waves are of 

 that simplest type which can be represented by a curve of sines — the curve 

 which represents the small oscillations of a pendulum. Eut when the waves 

 are of a more complicated form, two or more terms, perhaps all the terms, 

 of the series must be retained in order to represent it ; and in such cases 

 the terms of the series which remain severally represent the simple sinusoid 

 or pendulous undulations, which, if made to coexist in the medium by being 

 piled on one another, would become identical with the actual complex undu- 

 lation which is present in it. 



The non-periodic disturbances which traverse a medium are of two kinds — 

 those which, like the clang of a bell, may be represented by a series consisting 

 of sinusoid terms with distinct periodic times, though in this case not 

 harmonically, or at least not all harmonically, related ; and those which can 

 be decomposed into sinusoid elements only under the condition that the 

 elementary undulations have periodic times which pass without hiatus into 

 one another. 



Now so long as light is propagated through what is called a vacuum, the 

 undulation, however complex, maintains its form unaltered at all distances 

 from the source of light ; for in vacuous spaces waves of different periods 

 advance at the same rate and directly forwards, and therefore the simple 

 component undulations which are represented by the several terms of a 



* A Map of Oscillation-frequencies is in preparation, and will be presented to the 

 British Association at the Meeting at Sheffield in 1879. 



