PART. II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. V. 623 



"These are the only two lines shown in the sky comparison plate within the spectrum obtained on 

 the zodiacal light. Thus in so far as spectra of such low dispersion and resolving power can be trusted, 

 we would seem to have good evidence to support the claim that the zodiacal light is reflected sunlight". 



After these results we must ask: Is it conceivable that radiant matter can reflect sunlight as we 

 have supposed in our theory of the zodiacal light? 



Although analogies may often be misleading, there will undoubtedly be a certain value in the 

 recollection that the atmosphere, even in quite clear weather, diffuses the daylight to so great an extent 

 that even the most powerful stars are invisible. The light is sent back either from the air-molecules 

 themselves, or from microscopic dust-particles that are found in the atmosphere, as by the blue of the sky. 



Physical investigations of the power of electrically luminous gases to absorb and diffuse sunlight, 

 have not, as far as I am aware, been made on any large scale; but during the last few years some 

 very interesting results have been obtained, which will be discussed in these pages. With regard to 

 direct experimental research into the properties of radiant matter in the above respects, I do not think 

 anything has been ascertained. 



In the meanwhile, I have made some observations at Kaafjord in Finmarken, which will possibly 

 afford us some guidance in the question. 



1 have in broad daylight and at times in sunshine been able to observe rapidly-changing "clouds" 

 formed like draperies with radiant structure appearing at that time of the evening in which, in winter, 

 corresponding draperies of aurora are frequently seen. 



I have thought that these must be, not real clouds, but auroral rays scattering the sunlight and 

 therefore appearing like clouds. At all events it seems to me little likely that the condensation of 

 moisture could take place so rapidly in the highest regions of the atmosphere, and a moment afterwards 

 revert to vapour again (see page 450). 



I have found in literature certain investigations by R. LADENBURG and R. W. WOOD, of the optical 

 conditions in electrically luminous gases and in vapour, which are of great importance to the questions 

 we here touch upon. LADENBURG, in a treatise entitled "Ueber Absorption und Magnetorotation in 

 leuchtendem Wasserstoff "O), demonstrates that the number of absorbent "dispersion-electrons" is pro- 

 portional to the amplitude of the transfluent current. Now the intensity of the light is also proportional to that 

 ot the current, and the number of ions at constant pressure is proportional to the strength of the current. 

 All this should confirm the hypothesis that the bearer of the spectral hydrogen-series is the positive atomion. 



WOOD, after a number of interesting investigations of "Die vollstandige Balmersche Serie im Spektrum 

 des Natriums"(-),"Die selective Reflexion monochromatischcn Lichtes an Quecksilberdampf"( 3 ), and "TheUltra- 

 violet Absorption, Fluorescence and Magnetic Rotation of Sodium Vapour"( 4 ), is of opinion that the Balmer lines 

 and the accompanying spectra are produced by atoms that have lost one, two, three, four, and so on, electrons. 



There is now certainly very good reason for supposing that in the radiant matter which we assume 

 to have been radiated from the sun, there is comparatively a very large number of dispersion-electrons 

 that can take up and be in resonance with light-waves from the sun, and that possibly here too, this 

 number of dispersion-electrons is proportional to the enormous electric current-intensity that emanates from 

 the sun in (lie. manner here assumed. 



It will perhaps after this no longer be considered improbable that the mighty strata of radiant 

 matter we have imagined we could see into when we observe zodiacal light, are capable of diffusing suffi- 

 cient sunlight to occasion this slight brightness in the sky. Subsequent spectroscopic investigations may 

 possibly prove that the zodiacal light also contains a weak light of its own, which some observers have 

 thought to show, and thus does not merely reflect sunlight. 



(') Physikalische Zeitschrift. 10 Jahrgang, 1909; p. 497. 



(-> . p. 89. 



I 3 ' . . p. 45. 



I 4 ' ., . p. 913- 



