APPENDIX. iii 



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 ques- 

 tion of density, and of mode of illumination, as well as, in its 

 proper connection, 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, 

 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 result of the spectrum-analysis; which 

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

 rarity of this same material* The same rarity is withal indi- 

 cated by the transparency of the material. Of this Rev. George 

 Jones says, under date of Dec. 30, 1854 (in lat. 10^ 46' N., long. 

 89° 81' W. of Greenwich), "I also this morning gave attention 

 to the stars as seen through the zodiacal light, and found even 

 at 4'' 30™, when the effulgent light below the zigzag" (on the 

 chart) "is very strong, that, with the naked eye, I could readily 

 make out stars of the sixth magnitude within the effulgent light, 

 .... also a line of four stars below 19 Libra .... the two 

 northernmost of these last are of the seventh magnitude, yet I 

 think the naked eye detected them 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. "f 



The consideration of these phenomena inclines to the conclusion 

 that the zodiacal light proceeds from particles wjiich are molecu- 

 lar (at least in size), and not from discrete, though very small 

 solids. It then must also, largely, be transmitted light; and so 

 the illuminated material appear brighter in conformity with the 

 special direction in which the light is transmitted. Chaplain 

 Jones illustrates this in part — though he refers the light almost 

 entirely to reflection — when he says: "It seems to be quite con- 

 clusive, on an inspection of these charts, that ive never at any one 

 time see the whole actual extent of the zodiacal light.'''' This can 

 perhaps be elucidated by noticing a common event — a cloud sil- 

 vered 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 part of the border, an equally brilliant, 

 silvery light. But our eye is in a position to catch this reflection 

 from "only one portion of it, and the rest is dull to our vision. 

 If we could, with great rapidity, change our positions, other por- 

 tions of the silvered edge would show themselves, according to our 

 changes of place. So, also, when a rainbow is presented to our 

 eye ; the myriads of drops of falling water, in the whole rain- 



* Such was, in effoct, the statement of Prof. Charles A. Young (as the 

 result (if his experience and that of others), iu a personal couimunicatioa 

 with tlie author of this paper. 



t Vol. Ill of '-Report of Japan Expedition," No. 371, at p. 542. 



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