ZODIACAL LIGHT. 563 



the light it might almost be called illumination increased 

 surprisingly the more I approached the equator in South Ame- 

 rica and the South Sea. In the continually dry, clear air of 

 Cumana, in the grass-steppes (llanos) of Caracas, upon the 

 elevated plains of Quito and the Mexican seas, especially 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 forepart of 

 Argus and Sagittarius, or, to speak of our part of the hemis- 

 phere, 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 elevation of 

 the point whence it was seen, but much rather to depend prin- 

 cipally upon the interior variability of the phenomenon 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 r , since I do not 

 deny the possibility of a simultaneous influence of the condi- 

 tion of the air (greater or less diaphanity) of the higher 

 strata of the atmosphere, while my instruments indicated in 

 the lower strata, no hygrometric variations, or much rather 

 favourable 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 different elevation and 

 under different local conditions, would, with the application 

 of the theory of probabilities, decide what should be ascribed 

 to cosmical light-processes, what to merely meteorological 

 influences. 



It has been repeatedly affirmed that in Europe scarcely any 

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



VOL. IV. T 



