CHAPTER V. 



IS IT POSSIBLE TO EXPLAIN ZODIACAL LIGHT, COMETS' TAILS, AND 

 SATURN'S RING BY MEANS OF CORPUSCULAR RAYS? 



119. Zodiacal Light. In several of the experiments with a phosphorescent terella with different 

 screens, in a large discharge-tube, we have come upon phenomena which appeared capable of serving as 

 starting-points for an explanation of Zodiacal light. 



Zodiacal light is the name given to a brightness which appears in the western sky after sunset, 

 and in the eastern before sunrise, nearly following the line of the ecliptic in the heavens, and stretching 

 upwards to various altitudes according to the season of the year. 



Moreover, at certain periods of the year, what is called "Gegenschein" (discovered by BRORSEN), 

 occurs almost directly opposite to the position of the sun. 



Accurate observations have now shown that the axis of the zodiacal light diverges somewhat 

 noticeable from the ecliptic, and recent work has assumed that it is rather a question of the sun's 

 equator, than of the ecliptic. 



The great cosmologist, CASSINI, concluded after only ten observations the first detailed obser- 

 vations ever made that the axis of the zodiacal light has a relation to the sun's equator, rising and 

 sinking with it. 



Before I proceed further with the elucidation of this question, I will here mention a peculiarity of 

 the zodiacal light, which no attempt has ever been made to explain in anything approaching a satisfactory 

 manner by the various theories that have been advanced. This is a pulsation in the intensity and 

 shape of the light which has at times been noticed, a pulsation which surely testifies to an electric origin; 

 and I am therefore of opinion that the phenomenon is akin to the pulsation which is sometimes seen in 

 auroral lights and the oscillations in terrestrial magnetism. 



HUMBOLDT writes: "I have occasionally been astonished in the tropical climates of South America, 



to observe the variable intensity of the zodiacal light When the zodiacal light had been most 



intense I have observed that it would be perceptibly weakened for a few minutes, until it again sud- 

 denly shone forth in full brilliancy" (Cosmos, vol. I). 



Mr. BIRT, Kew Observatory, noticed in March, 1850, "One evening there was a sudden brightening 

 of the light for an instant, and also variations in its lustre of an intermittent character. These inter- 

 missions of brightness were observed on the same evening by Mr. LOWE at Nottingham" (Am. Journ. of 

 Sc., XV, second series, p. 121). 



The Rev. GEORGE JONES, a most diligent observer of zodiacal light, relates in March, 1854: "I was 

 surprised, one evening, at seeing the zodiacal light fade sensibly away, dimmed to almost nothing, and 

 then gradually brighten again. This was repeated several times; but the effect, after all, was to leave 

 me only in amazement and doubt. Subsequent nights, however, gave abundant exhibitions of this kind, 

 of which, with the times and changes, I have made ample records with the particularity that the case 

 required. 



