6 REPORT — 1891. 



just before their final completion, have assumed the character of a memo- 

 rial of him ; maps by Dr. Becker ; and sets of photographs of a high and 

 a low sun by Mr. McClean. 



At the meeting of this Association in Bath, M. Janssen gave an 

 account of his own researches on the terrestrial lines of the solar spec- 

 trum, which owe their origin to the oxygen of our atmosphere. He 

 discovered the remarkable fact that while the intensity of one class of 

 bands varies as the density of the gas, other diffuse bands vary as the 

 square of the density. These observations are in accordance with the 

 work of Bgoroff and of Olszewski, and of Liveing and Dewar on condensed 

 oxygen. In some recent experiments Olszewski, with a layer of liquid 

 oxygen thirty millimetres thick, saw, as well as four other bands, the 

 band coincident with Fraunhofer's A ; a remarkable instance of the 

 persistence of absorption through a great range of temperature. The 

 light which passed through the liquid oxygen had a light blue colour 

 resembling that of the sky. 



Of not less interest are the experiments of Knut Angstrom, which 

 show that the carbonic acid and aqueous vapour of the atmosphere reveal 

 their presence by dark bands in the invisible infra-red region, at the 

 positions of bands of emission of these substances. 



It is now some thirty years since the spectroscope gave us for the 

 first time certain knowledge of the nature of the heavenly bodies, and 

 revealed the fundamental fact that terrestrial matter is not peculiar to 

 the solar system, but is common to all the stars which are visible 

 to us. 



In the case of a star such as Capella, which has a spectrum almost 

 identical with that of the sun, we feel justified in concluding that the 

 matter of which it is built up is similar, and that its temperature is also 

 high, and not very different from the solar temperature. The task of 

 analysing the stars and nebula becomes, however, one of very great diffi- 

 culty when we have to do with spectra differing from the solar type. 

 We are thrown back upon the laboratory for the information necessary 

 to enable us to interpret the indications of the spectroscope as to the 

 chemical nature, the density and pressure, and the temperature of the 

 celestial masses. 



What the spectroscope immediately reveals to us are the waves which 

 were set up in the ether filling all interstellar space, years or hundreds 

 of years ago, by the motions of the molecules of the celestial substances. 

 As a rule it is only when a body is gaseous and sufficiently hot that the 

 motions within its molecules can produce bright lines and a corresponding 

 absorption. The spectra of the heavenly bodies are indeed to a great 

 extent absorption spectra, but we have usually to study them through 

 the corresponding emission spectra of bodies brought into the gaseous 

 form and rendered luminous by means of flames or of electric dis- 



