ZODIACAL LIGHT. 143 



ner ? An assumption of the existence of such meteorological 

 causes on the confines of our atmosphere is strengthened by 

 the " sudden flash and pulsation of light," which, according 

 to the acute observations of Olbers, vibrated for several sec- 

 onds through the tail of a comet, v/hich appeared during the 

 continuance of the pulsations of light to be lengthened by sev- 

 eral degrees, and then again contracted.* As, however, the 

 separate particles of a comet's tail, measuring millions of miles, 



* "What you tell me of the changes of light in the zodiacal light, 

 and of the causes to which you ascribe such changes within the trop- 

 ics, is of the greater interest to me, since I have been for a long time 

 past particularly attentive, every spring, to this phenomenon in our 

 northern latitudes. I, too, have always believed that the zodiacal light 

 I'otated ; but I assumed (contrary to Poisson's opinion, w^hich you have 

 communicated to me) that it completely extended to the Sun, with 

 considerably augmenting brightness. The light circle which, in total 

 solar eclipses, is seen suiTOunding the darkened Sun, I have regarded 

 as the brightest portion of the zodiacal light. I have convinced my 

 self that this light is very different in different years, often for several 

 successive years being very bright and diffused, while in other years 

 it is scarcely perceptible. I think that I find the first trace of an allu- 

 sion to the zodiacal light in a letter from Rothmann toTycho, in which 

 he mentions that in spring he has observed the twilight did not close 

 until the sun was 24*^ below the horizon. Rothmann must certainly 

 have confounded the disappeai'ance of the setting zodiacal light in the 

 vapors of the western horizon with the actual cessation of twilight. I 

 have failed to observe the pulsations of the light, probably on account 

 of the faintness with which it appears in these countries. You are, 

 however, certainly right in ascribing those rapid vainalions in the light 

 of the heavenly bodies, which you have perceived in tropical climates, 

 to our own atmosphere, and especially to its higher regions. This is 

 most strikingly seen in the tails of large comets. We often observe, 

 especially in tlie clearest weather, that these tails exhibit pulsations, 

 commencing from the head, as being the lowest part, and vibrating in 

 one or two seconds tln-ough the entire tail, which thus appears rapidly 

 to become some degrees longer, but again as rapidly contracts. That 

 these undulations, which were formerly noticed with attention by 

 Robert Hooke, and in more recent times by Schroter and Chladni, do 

 not actnally occur in the tails of the comets, but are produced by our at- 

 mosphere, is obvious when we recollect that the individual parts of 

 those tails (which ai'e many millions of miles in length) lie at very dif- 

 ferent distances from us, and that the light from their extreme points 

 can only reach us at intervals of time which differ several minutes from 

 one another. Whether what you saw on the Orinoco, not at intervals 

 of seconds, but of minutes, were actual coruscations of the zodiacal 

 light, or whether they belonged exclusively to the upper strata of our 

 atmosphere, I will not attempt to decide ; neither can I explain the 

 remarkable lightness of whole nights, nor the anomalous augmentation 

 and prolongation of the twilight in the year 1831, particularly if, as has 

 been remarked, the lightest part of these singular twilights did not coin- 

 cide with the Sun's place below the horizon." (From a letter wr'tten 

 by Dr. Olbers to myself, and dated Bremen, March 26th. 1833.) 



