10 REPORT 1891. 



body which may have a motion of approach or of recession of unknown 

 amount. 



But even here we are met by the confusion introduced by multiple 

 spectra, corresponding to different molecular groupings of the same 

 substance ; and, further, to the influence of substances in vapour upon 

 each other ; for when several gases are present together, the phenomena 

 ■of radiation and reversal by absorption are by no means the same as if 

 the gases were free from each other's influence, and especially is this the 

 <;ase when they are illuminated by an electric discharge. 



I have said as much as time will permit, and I think indeed sufl&cient, 

 to show that it is only by the laborious and slow process of most 

 cautious observation that the foundations of the science of celestial 

 physics can be surely laid. We are at present in a time of transition 

 when the earlier, and, in the nature of things, less precise observations 

 are giving place to work of an order of accuracy much greater than was 

 formerly considered attainable with objects of such small brightness as 

 the stars. 



The accuracy of the earlier determinations of the spectra of the 

 terrestrial elements is in most cases insufficient for modern work on the 

 stars as well as on the sun. They fall much below the scale adopted in 

 Rowland's map of the sun, as well as below the degree of accuracy attained 

 at Potsdam by photograpliy in a part of the spectrum for the brighter stars. 

 Increase of resolving power very frequently breaks up into groups, in the 

 spectra of the sun and stars, the lines which had been regarded as single, 

 and their supposed coincidences with terrestrial lines fall to the ground. 

 For this reason many of the early conclusions, based on observations as 

 good as it was possible to make at the time with the less powerful spec- 

 troscopes then in use, may not be found to be maintained under the 

 much greater resolving power of modern instruments. 



The spectroscope has failed as yet to interpret for us the remarkable 

 spectrum of the Aurora Borealis. Undoubtedly in this phenomenon 

 portions of our atmosphere are lighted up by electric discharges ; we 

 should expect, therefore, to recognise the spectra of the gases known to 

 be present in it. As yet we have not been able to obtain similar spectra 

 from these gases artificially, and especially we do not know the origin of 

 the principal line in the green, which often appears alone, and may have 

 therefore an origin independent of that of the other lines. Recently the 

 suggestion has been made that the Aurora is a phenomenon produced by 

 the dust of meteors and falling stars, and that near positions of certain 

 auroral lines to lines or flutings of manganese, lead, barium, thallium, iron, 

 .&c., are sufficient to justify us in regarding meteoric dust in the atmosphere 

 as the origin of the auroral spectrum. Liveing and Dewar have made a 

 •conclusive research on this point, by availing themselves of the dust of 

 excessive minuteness thrown off' from the surface of electrodes of various 



