1 6 L. VEGARD. M.-N. Kl. 



must have an atmosphere and a magnetic field, but the question then arises 

 as to why the other planets have not such dust rings? Several circum- 

 stances might prevent their formation as e. g. 



i) The temperature may be too high for the formation of clusters. 



2) The atmosphere might consist of Hydrogen and Helium which might 

 have too low a freezing point for the formation of clusters. 



3) The planet might have no atmosphere. 



4) The planet might have too weak a magnetic field. 



I shall pass these suggestions on to the astronomers. 



There are, however, some other points in connection with this view 

 with regard to the constitution of the upper strata of the atmosphere, which 

 call for considerable attention. 



We have already mentioned that the dust atmosphere scatters the light. 

 If the particles have a suitable average size the light might be selectively 

 scattered, and the upper atmosphere, when illuminated by the sun, would 

 seem to give out light of some colour. — In this way I think we may explain 

 the blue sky. 



The blue sky then sJiould be formed through the scattering of sioilight 

 in the Nitrogen dusters ivhich form the upper strata of the atmosphere. 



As alread}' mentioned a most characteristic feature of the northern 

 light spectrum is the great reduction in the number of lines. A few of the 

 N-lines are ver}' strong, and a few new ones set in — but most of the 

 lines are suppressed. 



Now we know that also spectra of Nebulae have a similar character; 

 they only consist of a few lines, many of which are not found among 

 the lines observed in laboratories. This similarity would suggest a similar 

 constitution of the Nebulae and the upper strata of the atmosphere, and, if 

 our view with regard to constitution of the auroral region is the right one, 

 it would suggest that the Nebulae consisted of fine dust particles made 

 luminous through the bombardment of electric rays. — These electric rays 

 might be due to disintegration of radio-active matter. — If the average 

 density of the Nebulae is very small, the temperature in spite of some radio- 

 activity may be very low, just as we have supposed to be the case in the 

 higher strata of the atmosphere. 



§6. 



In the first communication it was pointed out that we might also get 

 a sufficient density at a height of 6 — 700 km. by assuming a sufficiently 

 high temperature. 



In this case we should be able to apply the gas laws also for the 

 upper regions of the atmosphere, and, to calculate the density at various 

 altitudes, we should have to suppose that the temperature above a certain 



