ZODIACAL LIGHT. 133 



distant from the earth, it is not possible, according to the laws 

 of the velocity and transmission of light, that we should be 

 able, in so short a period of time, to perceive any actual 

 changes in a cosmical body of such vast extent. These con- 

 siderations in no way exclude the reality of the changes that 

 have been observed in the emanations from the more con- 

 densed envelopes around the nucleus of a comet, nor that of 

 the sudden irradiation of the zodiacal light from internal mo- 

 lecular motion, nor of the increased or diminished reflection of 

 light in the cosmical vapour of the luminous ring ; but should 

 simply be the means of drawing our attention to the differences 

 existing between that which appertains to the air of heaven 

 (the realms of universal space), and that which belongs to the 

 strata of our terrestrial atmosphere. It is not possible, as 

 well-attested facts prove, perfectly to explain the operations 

 at work in the much- contested upper boundaries of our at- 

 mosphere. The extraordinary lightness of whole nights in 

 the year 1831, during which small print might be read at 

 midnight in the latitudes of Italy and the North of Germany, 



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

 ever, certainly right in ascribing those rapid variations 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 strik- 

 ingly seen in the tails of large comets. We often observe, especially in the 

 clearest weather, that these tails exhibit pulsations, commencing from the 

 head, as being the lowest part, and vibrating in one or two seconds through 

 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 hi more recent times by 

 Schroter and Chladni, do not actually occur in the tails of the comets, 

 but are produced by our atmosphere, is obvious when we recollect that 

 the individual parts of those tails (which are many millions of miles in 

 length), lie at very different 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, T 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 coincide with 

 the Sun's place below the horizon." (From a letter written by Dr. Olbers 

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



