PORTION OF THE COSMOS. ZODIACAL LIGHT. 417 



ever, must not be limited to the configuration of the light, 

 derived either from distances from known stars, or from 

 direct measurements. The intensity of the light, its unifor- 

 mity, or, on the other hand, its intermittence (quivering and 

 flashing), and its analysis by the polariscope, ought to be 

 the chief objects of examination. Arago (in the Annuaire 

 for 1836, p. 298) has already pointed out that a compari- 

 son of the observations of Dominique Cassini is perhaps 

 sufficient to show " que la supposition des intermittences 

 de la diaphanite atmospherique ne saurait sum* re a 1'explica- 

 tion des variations signalees par cet astronome." 



Immediately after the first Paris observations of this great 

 observer, and of his friend, Patio de Duillier, the Zodiacal 

 Light attracted the regard of the Indian voyagers, Pater 

 Noel, de Beze, and Duhalde > but detached notices (for the 

 most part chiefly occupied with describing the gratification 

 afforded by the unwonted spectacle) are not available for a 

 thorough discussion of the causes on which the variability of 

 the light depends. It is not on rapid journeys, or voyages 

 called voyages of circumnavigation, as the endeavours of the 

 active Horner have shewn in more recent times (Zach, Mo- 

 natl. Corresp. Bd. x. S. 337340), that the desired object 

 can be attained in a thorough and satisfactory manner. A 

 permanent residence of several years in some of the countries 

 of the tropics is requisite for obtaining the solution of the 

 problems, presented by the variations in form and intensity 

 of the Zodiacal Light. Tor this object, as well as for me- 

 teorology generally, the greatest advantages may be expected, 

 when scientific cultivation shall at length have extended 

 over the equinoctial regions formerly called Spanish America, 



