The solar corona of a sun-spot maximum year is of a very different appearance (Fig. 31). The streamers radiate straight from the sun in almost all directions; and if there be some privileged directions, it will be those above the sun-spot belts. The velocity of the solar dust is evidently so great that the streamers are no longer visibly deflected by the magnetic lines of force of the sun. Nor is this charged dust influenced to any noticeable degree by the magnetic lines of force of the earth. It will in the main fall straight down in that part of the atmosphere in which the radiation is most intense: As these “hard” rays of the sun[8] seem to issue from the faculæ of the sun which are most frequent in maximum sun-spot years, some polar lights will also be seen in districts which are far removed from the maximum belt of the auroras, especially when the number of sun-spots is large. The opposite relation holds for the “soft” streams of solar dust which fall near the maximum belt of the polar lights. These streams occur most frequently with low sun-spot frequency, as we know from observations of the solar corona. Possibly they are carried along by the stream of harder rays in maximum years. The polar lights corresponding to these rays therefore attain their maximum with few sun-spots. Hard and soft dust streams occur, of course, simultaneously; but the former predominate in maximum sun-spot years, the latter in minimum years. [Footnote 8: The designations “hard” and “soft” streams of solar dust correspond to the terms used with regard to kathode rays. The soft rays have a smaller velocity, and are therefore more strongly deflected by external forces, as, for instance, magnetic forces.] That the periodicity of the polar lights in regions without the maximum belt follows very closely the periodicity of the sun-spots was shown by Fritz as early as 1863. The length of the period varies between 7 and 16 years, the average being 11.1 years. The years of maxima and minima for sun-spots and for northern auroras are the following: MAXIMUM YEARS Sun-spots 1728 ’39 ’50 ’62 ’70 ’78 ’88 1804 ’16 ’30 1837 ’48 ’60 ’71 ’83 ’93 1905 Northern lights 1730 ’41 ’49 ’61 ’73 ’78 ’88 1805 ’19 ’30 1840 ’50 ’62 ’71 ’82 ’93 1905 MINIMUM YEARS Sun-spots 1734 ’45 ’55 ’67 ’76 ’85 ’98 1811 ’23 ’34 1844 ’56 ’67 ’78 ’89 1900 Northern lights 1735 ’44 ’55 ’66 ’75 ’83 ’99 1811 ’22 ’34 1844 ’56 ’66 ’78 ’89 1900