502 SCIENTIFIC RECREATIONS. 



observed by Schioparelli, that the light is much more difficult to make out 

 when it passes through the meridian, than when it is only 30 above the 

 horizon, and is less easily seen when the air is clearest than when a sort 

 of mist is present. Indeed, the bright parts of the Milky Way may be seen 

 to be weakened by mist, while the zodiacal light at the same height is 

 unaffected. The zodiacal light has temporary variations of light intensity, 

 and it shows from time to time remarkable changes of form and position, so 

 sudden and short as to be hard to explain on the planetary hypothesis. 

 The elongations of the cones show a half-yearly period, which is independent 

 of the transparency of the air. The cone follows the observer northwards or 

 southwards, so that there is no parallactk action ; and this peculiarity (so 

 adverse to the extra-terrestrial hypothesis) cannot be explained by reflection 

 or absorption of light. As to spectroscopic observations, the author finds 

 (i) that the zodiacal light consists partly of proper light ; (2) that its con- 

 nection with polar light is but secondary, temporary, and accidental ; (3) 



Fig. 552. On the sun's disc. 



that the cause of the second phenomenon is such that it may strengthen the 

 zodiacal light and modify its spectrum ; and (4) that the results of spectrum 

 analysis rank with other arguments tending to find the source of the zodiacal 

 light in the neighbourhood of the earth (like the polar light). Herr 

 Gronemann, then, thinks the zodiacal light a terrestrial phenomenon, though 

 he will not say that it cannot be influenced by cosmic action. He throws 

 out the suggestion that the cone may be a kind of optical illusion, arising 

 from some fine matter gas or dust being more accumulated near the 

 observer in one direction than another. The apparent length of the cone 

 might be conditioned by the conical shadow of the earth, and the changes of 

 length be due to cosmic and electric influences. In any case, there is need 

 of a more scientific theory than the old one. 



We may conclude our brief notice of the great luminary to which we 

 are indebted for everything, by a resume of his distance from us, his diameter, 

 and a few other plain facts and figures. In the first place the actual 

 distance of the sun from the earth has never accurately been determined, 



