Intelligence and Miscellaneous Articles. 79 



On high mountains, the rays of the solar spectrum which originate 

 in our atmosphere experience, during the course of the day, varia- 

 tions of intensity far more marked than in the plain; on the Faul- 

 horn I could recognize the telluric origin of important groups, the 

 discrimination of which hitherto was dubious. These groups, which 

 belong to the red end of the spectrum, are as follows : — 



1. A portion at least of Fraunhofer's ray B, which is the least 

 refrangible ; the intensity of the ray forming the other portion of 

 the spectrum does not enable one to state positively. 



2. The groups between B and a consist almost exclusively of tel- 

 luric rays. 



3. The group a is telluric ; that is, the red end of the spectrum 

 from B to A is ruled by rays which almost all originate in the ter- 

 restrial atmosphere. The importance of the telluric phenomenon 

 there is almost tenfold that of the solar phenomenon. 



I may remark here that M. Kirchhoff's maps for the entire region 

 from A to B exhibit no coincidence between the rays of the solar 

 spectrum and those of metals studied by this eminent physicist. 

 The discovery of the telluric origin of the groups of this region 

 explains this circumstance, and confirms its exactitude. 



If we cast a glance at the general distribution of the telluric 

 groups in the solar spectrum, we see that these groups are so much 

 more important and numerous as we consider the least refrangible 

 portion of the solar spectrum ; the exact opposite is the case for the 

 rays of solar origin. 



The memoir contains an account of an experiment made on the 

 lake of Geneva, between Lyons and Geneva, and in which I could 

 confirm the production of telluric groups in the spectrum of a flame 

 which at a small distance presented none. This experiment shows 

 directly the action of the elective absorption of our atmosphere. 



In reference to my former communication on this subject, Father 

 Secchi announced that he had observed an increased intensity in the 

 telluric bands on cloudy days, or when the atmosphere is whitish 

 and vapoury, or also, when the moon is viewed, veiled by the effect 

 of vapours. Secchi concluded from this observation, referred to that 

 of the spectra of the planets, the very probable existence of aqueous 

 vapour in the atmosphere of these stars. 



I observed on this occasion, that the results given by the analysis 

 of the light of clouds were in general too complicated to elucidate 

 a question of this kind. "When the atmosphere is slightly veiled 

 by white clouds, a given point of the heavens sends to the eye a 

 larger quantity of light than when the heavens are clear ; and this 

 light arises from multiple reflexions from the aqueous particles. 

 Under these conditions the spectrum obtained is more luminous ; 

 moreover it consists of rays which, in consequence of their numerous 

 reflexions, have traversed great thicknesses of the atmosphere. These 

 two conditions perfectly explain the easier and more marked vision 

 of the telluric bands which then take place. In this case the vapour 

 of the cloud has only served as reflector to send to the instrument 

 the rays which have traversed great thicknesses of the atmosphere ; 



