ZODIACAL LIGHT, 'iUb 



surprisingly the more I approached the equator in South 

 America and the South Sea. In the continually dry, clear 

 air of Cumana, in the grass-steppes [llanos) of Caraccas, upon 

 the elevated plains of Q,uito and the Mexican seas, especial- 

 ly at heights from eight to twelve thousand feet, where I 

 could remain longer, the brightness sometimes exceeded that 

 of the most beautiful sparks of the Milky Way, between the 

 fore part of Argus and Sagittarius, or, to speak of our part of 

 the hemisphere, between the Eagle and the Swan. 



Upon the whole, the brightness of the zodiacal light did 

 not appear to me to increase at all perceptibly with the ele- 

 vation of the point whence it was seen, but much rather to 

 depend principally upon the interior variability of the phe- 

 nomenon itself — upon the greater or less intensity of the 

 light-giving process, as is shown by my 'observations in the 

 South Sea, in which, indeed, a reflection was remarked like 

 that seen on the going down of the Sun. I say principally, 

 since I do not deny the possibility of a simultaneous influence 

 of the condition of the air (greater or less diaphaneity) of the 

 higher strata of the atmosphere, while my instruments indi- 

 cated in the lower strata no hygrometric variations, or, much 

 rather, favorable ones. Advances of our knowledge of the 

 zodiacal light are to be expected especially from the tropics, 

 where the meteorological processes attain the "highest degree 

 of uniformity or regularity in the periodical recurrence of the 

 changes. The phenomenon is there perpetual ; and a careful 

 comparison of observations at points of diflerent elevation and 

 under diflerent local conditions, would, with the application 

 of the theory of probabilities, decide what should be ascribed 

 to cosmical light-processes, what to merely meteorological in- 

 fluences. 



It has been repeatedly aflirmed that in Europe scarcely 

 any zodiacal light, or only a feeble trace of it, could be seen 

 in several successive years. Has the light appeared propor- 

 tionately weakened in such years in the equinoctial zone 

 also ? The investigation must not, however, be restricted to 

 the statement of the configuration according to the distance 

 /rom known stars or direct measurements. The intensity of 

 the light, its uniformity or probable intermittence (darting 

 and flashing), its analysis by the polariscope, should be espe- 

 cially investigated. Arago [Annuaire for 1836, p. 289) has 

 •,lready pointed, out that the comparative observation of Dom- 

 inique Cassini would perhaps clearly prove "que la supposi- 

 tion des intermittences de la diaphanite atmospherique ne 



