The quantity of solar dust which reaches our atmosphere will naturally vary in proportion with the eruptive activity of the sun. The quantity of dust in the higher strata influences the color of the light of the sun. After the eruption of the volcano Rakata on Krakatoa, in 1883, and again, though to a lesser degree, after the eruption of Mont Pelée on Martinique, red sunsets and sunrises were observed all over the globe. At the same time, another phenomenon was noticed which could be estimated quantitatively. The light of the sky is polarized with the exception of the light coming from a few particular spots. Of these spots, one called Arago’s Point is situated a little above the antipode of the sun, and another, Babinet’s Point, is situated above the sun. If we determine the elevation of these points above the horizon at sunset, we find in accordance with the theoretical deduction that this elevation is greater when the higher strata of the atmosphere are charged with dust (as after the eruption of Rakata) than under normal conditions. Busch, a German scientist, analyzed the mean elevation of these points (stated in degrees of arc) at sunset, and found the following peculiar numbers: │1886 │ ’87 │ ’88 │ ’89 │ ’90 │ Arago’s Point │20.1 │19.7 │18.4 │17.8 │=17.7=│ Babinet’s Point│23.9 │21.9 │17.9 │56.8 │=15.4=│ Sun-spot Number│21.1 │19.1 │ 6.7 │=6.1=│ 6.5 │ │ ’91 │ ’92 │ ’93 │ ’94 │ ’95 │Mean Arago’s Point │20.6 │19.6 │ 20.2 │=20.7=│18.8 │19.4 Babinet’s Point│23.3 │21.5 │=24.2=│ 23.3 │19.0 │20.7 Sun-spot Number│35.6 │73.8 │=84.9=│ 78.0 │63.9 │40.0 There is a distinct parallelism in these series of figures. Almost simultaneously with the sun-spot maximum the height of the two so-called neutral points above the horizon attains its maximum at sunset, and the same applies to the minimum. That the phenomena in the atmosphere take place a little later than the phenomena on the sun which caused them is perhaps only natural. When the air is rich in dust, or when it is strongly ionized by kathode rays, conditions are favorable for the formation of clouds. This can be observed, for instance, with auroral lights. They regularly give rise to a characteristic cloud formation, so much so that Adam Paulsen was able to recognize polar lights by the aid of these clouds in full daylight. Klein has compiled a table on the connection between the frequency of the higher clouds, the so-called cirrus clouds, at Cologne, and the number of sun-spots during the period 1850-1900. He demonstrates that during this half-century, which comprises more than four sun-spot periods, the sun-spot maxima fell in the years in which the greatest number of cirrus clouds had been observed. The minima of the two phenomena are likewise in agreement.