of the Zodiacal Light. 19 



Smyth*, and others, which show that the spectrum is continuous, 

 and not perceptibly different from that of faint sunlight. The 

 writer has also made numerous observations with a spectroscope 

 specially arranged for faint light, of which an account will be 

 published hereafter, and which lead to the same conclusion. It 

 may be mentioned further that a particular object in these ob- 

 servations was to determine whether any bright lines or bands 

 w T ere present in the spectrum, or whether there is any connexion 

 between the zodiacal light and the polar aurora ; and the results 

 give, as an answer to the question, a decided negative. This is 

 important here, as excluding from the possible causes of the 

 light the luminosity of gaseous matter, either spontaneous or 

 due to electrical discharge. The supposition that the light is 

 reflected from masses of gas, or from globules of precipitated 

 vapour, is not to be entertained, since, as Zollnerf has shown, 

 such globules in otherwise empty space must evaporate com- 

 pletely, and a gaseous matter would expand until its density 

 became far too small to exert any visible effect upon the rays of 

 light. 



We must conclude, then, that the light is reflected from mat- 

 ter in the solid state — that is, from innumerable small bodies 

 revolving about the sun in orbits, of which more lie in the 

 neighbourhood of the ecliptic than near any other plane passing 

 through the sun. Although such a cause for the zodiacal light 

 has often been assumed as probable, no satisfactory proof of it 

 has hitherto been found ; and the establishment of the fact of 

 polarization was necessary to its confirmation, since spectroscopic 

 appearances alone leave it uncertain whether the matter is not 

 self-luminous. 



If these meteoroids, as there is no good reason to doubt, are 

 similar in their character to those which have fallen upon the 

 earth, they must be either metallic bodies, chiefly of iron, or 

 stony masses with more or less crystalline structure and irre- 

 gular surfaces. If we accept Zollner's conclusion that the 

 gases of the atmosphere must extend throughout the solar sys- 

 tem, though in an extremely tenuous condition in space, the 

 oxidation of the metallic meteoroids would be merely a question 

 of time. They would thus become capable of rendering the 

 light reflected from them plane-polarized ; and the same effect 

 would in any case be produced by those of the stony character. 



In order to ascertain whether the proportion of polarized light 

 actually observed approached in any degree what might be ex- 

 pected from stony or earthy masses of a semicrystalline cha- 

 racter with a granular structure and surfaces more or less 



* Monthly Notices of the Royal Astronomical Society, June 18/2, p. 277- 

 t Ueber die Natur der Cometen, p. 7^ et seq. 



C2 



