132 COSMOS. 



and wavering of the light. Must we suppose that changes 

 are actually in progress in the nebulous ring ? or is it not 

 more probable, that although I could not, by my meteorolo- 

 gical instruments, detect any change of heat or moisture near 

 the ground, and small stars of the fifth and sixth magnitudes 

 appeared to shine with equally undiminished intensity of 

 light, processes of condensation may be going on in the 

 uppermost strata of the air, by means of which the trans- 

 parency, or rather the reflection of light, may be modified in 

 some peculiar and unknown manner ? An assumption of the 

 existence of such meteorological causes on the confines of our 

 atmosphere, is strengthened by the " sudden flash and pulsa- 

 tion of light," which, according to the acute observations of 

 Olbers, vibrated for several seconds through the tail of a 

 comet, which appeared during the continuance of the pulsa- 

 tions of light to be lengthened by several degrees, and then 

 again contracted. As, however, the separate particles of a 

 comet's tail, measuring millions of miles, are very unequally 



* Arago, in the Annuaire, 1832, p. 246. Several physical facts appear 

 to indicate that, in a mechanical separation of matter into its smallest 

 particles, if the mass be very small in relation to the surface, the electrical 

 tension may increase sufficiently for the production of light and heat. Ex- 

 periments with a large concave mirror, have not hitherto given any positive 

 evidence of the presence of radiant heat in the zodiacal light. (Lettre 

 de M. Matthiessen a M. Arago, in the Comptes Rendus, t. xvi. 1843, 

 Avril, p. 687.) 



f " 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 tropics, is of 

 the greater interest to me, since I have been for a long time past particu- 

 larly attentive, every spring, to this phenomenon in our northern latitudes. 

 I, too, have always believed that the zodiacal light rotated ; but I assumed, 

 (contrary to Poisson's opinion, which you have communicated to me,) that 

 it completely extended to the Sun, with considerably augmenting bright- 

 ness. The light circle which, in total solar eclipses, is seen surrounding 

 the darkened Sun, I have regarded as the brightest portion of the zodiacal 

 light. I have convinced myself that this light is very different in different 

 years, often for several successive years being very bright arid diffused, 

 whilst in other years it is scarcely perceptible. I think that I find the 

 first trace of an allusion to the zodiacal light in a letter from Rothmann to 

 Tycho, 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 disappearance of the setting zodiacal light 

 in the vapours of the western horizon, with the actual cessation of twilight. 

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



