A.— MATHEMATICAL AND PHYSICAL SCIENCES. 45 



sector or a wire gauze not seen in focus. The astronomical equivalent of 

 those devices is a swarm of meteorites, and it may be necessary to invoke 

 their aid, but the rare gaseous atmosphere required to give the line spectrum 

 of hydrogen, helium, nitrogen and oxygen cannot be considered to blend 

 harmoniously with a swarm of meteorites or to have anything like a comple- 

 mentary relation to it , and it is particularly difficult to understand from 

 this point of view how it can come about that the bright nebular lines 

 are often seen on a profoundly dark background almost or quite free from 

 continuous spectrum. 



Comets. 



A kindred problem is that of the luminosity of comets. This has 

 been discussed by Zanstra in a recent paper. [M.N., Dec, 1928]. Ho 

 takes the view that the Swan bands of carbon are resonance bands excited 

 by light from the sun in the visual spectrum, the gases being at an ordinary 

 temperature such as prevails at the earth's surface. If these are really 

 the conditions, the problem of imitating the comet seems ideally easy 

 from the laboratory point of view. The Swan spectrum should appear in 

 absorption of suitable carbonaceous gases, contained in a vessel at the 

 ordinary temperature ; and it should be observable in lateral emission. 

 I cannot help thinking that if nothing more than this was necessary, the 

 thing would have been done before now. In the case of the D line sodium, 

 treated by Zanstra as quite analogous, it has of course been done long 

 ago in the phenomenon described by R. W. Wood, and called resonance 

 radiation. 



Metastable States. 



In discussing the nebular and auroral spectra, we encountered the idea of 

 ' metastable states.' At present this conception is not in a very satisfactory 

 condition. The original idea was of a state which did not allow of 

 direct transition by emission of radiation to the stable ordinary state. 

 Let us compare the level of the atom to the stories of a building and the 

 optical electron to a man inside the building. The ordmary state of the 

 atom is represented by the man being on the ground floor, and the meta- 

 stable state by placing him on the first floor. But the internal architecture 

 of our building must be pictured as peculiar. A staircase connects the 

 flrst floor with the second floor, and another staircase connects the second 

 floor with the ground floor : but there is no connection between the first 

 floor and the ground floor except by going up higher and coming down again. 



Such, I say, was the original conception, but facts which have since 

 come to light require some revision of it. 



In the nebulfe the electron manages somehow to escape from its prison- 

 house, and descend to the level below not by the legitimate route of 

 going upstairs and down again, but by illicitly breaking through the 

 floor, contrary to the rules of the establishment. 



Abandoning the metaphor, and the attempt to use popular language, 

 the selection rule which forbids transitions not involving a change in 

 the azimuthal quantum number is violated in all such cases. The 

 inner quantum number rule, which requires that the inner quantum 

 number should not change from 2 to or from to is also violated 

 in one class of cases, and rather meticulously observed in another. 



