376 G, Jones on the Zodiacal Light. 



being clearly visible, but still not perceptibly dimming tbe stars 

 beyond. My eye soon became accustomed to tracing it : and, 

 from that time on, I never failed to see this luminous arch, at 

 every hour of the night, when the moon or clouds did not prevent. 

 I soon perceived that a cross-section of it would not present the 

 same intensity of light quite across: but that it was brightest at 

 its central part or central longitudinal line, dimming thence to- 

 Avards the edges or boundaries, which latter I sometimes found 

 it difficult to make out. At the central part (running length- 

 wise) the line of strongest light was so well marked that I soon 

 began to draw it on my charts, where I consider it fully reliable, 

 showing both the inclination of this arch or ring to the ecliptic 

 and the places of the nodes. As a proof of its clearness of 

 marking among the stars, I may mention, that on the three dif- 

 ferent occasions, when other persons who had never before no- 

 ticed the Zodiacal Light came to observe with me, and I said to 

 them merely, ^'You see that luminous arch up in the sky:" 

 ''you see that, at its central part, the light is brighter than in 

 any other portion : now I want you to tell me by what stars that 

 brightest part passes," they never hesitated at once to give me 

 the place of that strongest light, and to give it almost or quite 

 exactly where I was in the habit of drawing it myself So they 

 did also with the boundaries of this luminous arch. Sometimes 

 the light did not pass, by a regular dimming from this central 

 line to the boundaries ;. but there was on each side a sudden 

 oiFset or decided dimming about half way from it to the outer 

 limits, whence it dimmed gradually again to the*boundari^. I 

 have repeated markings of this offset on my charts. It will be 

 readily seen that I was not only, once every night, on the eclip- 

 tic; but was, six hours from that, at a distance of 23 or 24 de- 

 grees from that line : when this occurred tow^ards the south, there 

 was, on that side of the arch, beyond its usual boundaries, a 

 faint and very dim light, as if I was then looking laterally at the 

 nebulous ring. I did not ever notice this on the northern side. 

 The light of this luminous arch was, at midnight, of a uniform 

 character from one horizon to the other ; a cold, white light, ex- 



. Way 



nniform width the whole wav across 



V 



\vliere it widened out slightly ; the nebulous substance at those 

 lower extremes doubtless giving me reflected light, by which its 

 outer edges of very diffuse matter were visible to the eye. Tb^ 

 last sentence will convey to the reader's mind the idea that I did 

 not consider the whole of this luminous arch as being reflected 

 light: and I certainly, at an early stage of my observations on 

 the Cordilleras, did begin to query whether this nebulous ring 

 was not giving me light independently of the sun : that is— 

 whether it was not self-luminous ! My reasons for suspectng 



