PART II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. V. 625 



It has subsequently appeared, after all the magnetic curves have been developed, that an unusual 

 magnetic calm has happened to prevail during all the times at which we obtained serviceable observations 

 of the light; there were hardly any perceptible magnetic changes at those times. Only on the 3oth 

 April did it appear that there had been a magnetic storm during the hours in which the observations of 

 the zodiacal light were made on my departure from Khartoum. 



Our attempts to obtain good photographs of the zodiacal light were at first without result. We 

 tried altogether five or six combinations of lenses, some of the lenses being very expensive. At last 

 \v<: succeeded, by telegraphic order, in obtaining from Cairo and Dresden some simple cinematograph 

 lenses, which gave fairly satisfactory results. 



We then took, both at Omdurman and Assouan, at exactly the same hours, two dozen plates each 

 evening during the last few days of our stay. 



The times were photographically recorded from an electrically illuminated watch upon each plate 

 at the beginning and end of each exposure. 



There is at present nothing more to say about our results here, but it was at any rate ascertained 

 that it was possible to obtain good photographs with our simple cinematograph lenses, by employing 

 HaufTs "Ultra-Rapid" plates, which ought by preference to be illuminated in before the exposure accord- 

 ing to Wood's method ( ] ). 



It is my intention as soon as possible to continue these investigations, perhaps with two stations, 

 in the Andes in South America. By photographing the zodiacal light simultaneously from two such 

 stations, it might be possible to obtain a parallax determination. According to HUMBOLDT, the conditions 

 there should be especially favorable, for in his "Cosmos", Vol. I, he remarks: "I have seen it shine with 

 an intensity of light equal to the Milky Way in Sagittarius". Judging from our photographs, this 

 should answer to an intensity of the zodiacal light from 5 to 10 times greater than that which we observed 

 in Egypt and Soudan. 



As we thus obtained a negative result with regard to the pulsations of the zodiacal light by our 

 observations, we determined instead to study the magnetic curves at Greenwich for the period during 

 which JONES had carried on his observations. This observatory is presumably the only one in which, as 

 early as 1853, continuous magnetic registerings were made. 



On going through JONES' observations, we find a considerable number of days on which he seems 

 to have noticed pulsations of light. On two occasions he is absolutely convinced of their existence, 

 namely, on the 3oth January, and the 2;th March, 1854. On tne first of these we read, in italics : 

 "There can be no doubt that there are pulsations in the zodiacal light"; on the second he remarks: 

 "It certainly does pulsate". 



The curves at Greenwich are drawn by instruments with great sensitiveness and comparatively long 

 time-periods, so that possible magnetic pulsations would be more easily discovered than by the ordinary 

 daily magnetograms. But the curves have been faint and have been gone over with ink, and have thus 

 lost something of their character. 



There are here reproduced four plates with magnetograms from Greenwich, first, two answering to 

 the above-mentioned dates, the 3oth January and the 2yth March, 1854, belonging to JONES, next, two 

 answering to the 2jth February and 25th April of the same year, when in JONES' observations too, 

 distinct pulsations are recorded. This comprises the most certain pulsations observed in the zodiacal light. 



We have further chosen 5 days with light-pulsations, for which we have copies of the curves at 

 Greenwich, which distinctly show magnetic pulsations simultaneously with those observed in the zodia- 

 cal light. 



(') Phys. Zeit. 1908, p. 355. 



