CERTAIN HARMONIES OF THE SOLAR SYSTEM. 53 



reflection from the earth's atmosphere is also discussed and rejected by him. 

 Upon this, however, it will not be necessary here to comment; as it, most proba- 

 bly, is no longer insisted upon by any one. 



(74) It remains, then, to consider loith loliat modifications we may admit Mr. 

 Jones's hypothesis ; that the nebulous material which gives the Zodiacal Light is a 

 terrestrial appendage; and also what is the conservative force, which may insure 

 its preservation of form, and its maintenance in its revolution around the earth, 

 even in close proximity to the moon. 



Antecedent to all that, however, will be found to be the questions of density 

 and of mode of illumination, as well as, in its proper connexion, the question of 

 parallax. 



The density of the material in question seems indeed to be that intimated in 

 the description of M. Laplace already quoted, (71); viz. that which pertains to the 

 state of molecules "too volatile either to combine themselves, or to unite with the 

 planets." And this is confirmed by the spectrum-analysis; the result of which 

 has led to no other reliable conclusion than that of the extreme rarity of this same 

 material.-^ 



This same rarity of the material in question is withal indicated by its trans- 

 parency. 



Of this Rev. George Jones says, under date of Dec. 30, 1854 (in lat. 10°46' N., 

 long. 89°3r W. of Greenwich): "I also, this morning, gave attention to the stars 

 as seen through the Zodiacal Light, and found, even to d*" 30™, when the effulgent 

 light below the zigzag lines (in the chart) is very strong, that with the naked eye 

 I could readily make out stars of the 6th magnitude within the effulgent light; 



also a line of four stars below 19 Librae, and ranging with j3 Librae; 



the two northernmost of these last are of the 7th magnitude, yet I think the 

 naked eye detected them, even within this effulgent light; but the last are near 

 its upper edge. All this shows the great transparency of the substance giving 

 the Zodiacal Light. "^ 



(75) The consideration of these phenomena leads to the conclusion, That this 

 light proceeds from particles which, as respects size, are, at most, all but molecular, 

 and if discrete, and, possibly, " solid," yet excessively small solids. It then must also 

 largely be transmitted light ; and so the illuminated material appear hrighter in the 

 special direction in tokich the light is transmitted. Chaplain Jones illustrates this in 

 part, when he says that " it seems to be quite conclusive, on an inspection of these 

 charts, that loe never at any one time see the tohole actwd extent of the Zodiacal Light. 

 This subject can, perhaps, be elucidated by noticing a common event — a cloud 

 silvered at one edge by the rays of the declining sun. The sun may be shining on 

 the bordering, quite around that cloud; and, if so, it is sending off from every por- 

 tion of the border, an equally brilliant silvery light. But our eye is in a position to 



' Sucb is in effect the statement of Prof. Charles A. Young (as the result of his experience and 

 that of others), made in a personal communication with the author of this paper. 

 ^ Report of Japan Expedition, vol. iii, No. 271, at p. 542. 



