Trof. Challis on the Zodiacal Light, 117 



If Dr. Draper advanced the true theory in 1840, it made no 

 impression on himself or on others. No scientific writer, to my 

 knowledge, either in America or Europe (and I have searched 

 far and wide for the purpose), has ever had a doubt that the 

 camphor deposits are produced by the action of light. Nor do 

 I now see any cause to alter one of the cod elusions of my paper, 

 viz. that the result of Dr. Draper's labours was " to multiply 

 phenomena, and to leave the theory as it was." 

 Apologizing for the length of this letter, 



I remain, &c, 

 King's College, London, C. ToMLINSON. 



Januarv 5, 1863. 



XV. A Theory of the Zodiacal Light. 

 By Professor Challis, M.A., F.R.S., F.R.A.S* 



THE following theory of the zodiacal light is the same as that 

 of which I gave an outline at the Meeting of the British 

 Association held in October last at Cambridge. I propose now 

 to produce it in more detail, and to exhibit the mathematical 

 reasoning by which it is sustained. So far as the facts to be 

 explained may be regarded as phenomena of light, reference will 

 be made for their explanation to those motions of an elastic fluid 

 medium which I have investigated in several recent communica- 

 tions to this Journal, and have especially applied in the Number 

 for last December in the explanation of various properties and 

 laws of light. That the zodiacal light, regarded merely as a 

 phenomenon of light, comes under the class of phenomena ex- 

 plained in that communication, which are distinguished from 

 those of reflexion, refraction, &c. by not being immediately 

 related to visible and tangible substances, will be made to appear 

 by reference to direct observations. There are other character- 

 istics of it, such as its form and position, the proposed explana- 

 tions of which depend on motions of the sethereal medium differ- 

 ent in kind from those which account for the properties of light. 

 The laws of these motions will be deduced by mathematical in- 

 vestigation. But it is necessary first of all to state distinctly 

 the facts and appearances which it is proposed to explain. 



In the northern hemisphere the zodiacal light is seen in the 

 spring months in the west, after the departure of twilight, as a 

 very faint light about 12 degrees broad at its base in the horizon, 

 converging to an apex, and having its axis nearly coincident with 

 the ecliptic. It is most conspicuous in the months of February 

 and March, at which season the apex is near the Pleiades. Similar 

 phenomena are visible before morning twilight in the east in the 



* Communicated by the Author. 



