INTRODUCTION. V 



the Royal Observatory at Paris, and doubtless the greatest cosmologist of his day. His first 

 notice of the Zodiacal Light was on the evening of the 18th of March, 1083, when, observing 

 in the west for other things, he was struck with this luminous streak reaching far up in the 

 sky. Unfortunately for subsequent times, Cassini's active mind immediately began to theorize; 

 and he had made but ten observations, when he worked up an hypothesis, which, though formed 

 so rapidly and on such slender and insufficient data, has yet, ever since, warped the judgment 

 of astronomers, and even led to imperfect modes of observing the phenomena of the Zodiacal 

 Light. We have, in this case, an exhibition of the danger of beginning to theorize before we 

 have a sufficient supply of data to work upon. 



Cassini discovered very soon that, as time advanced through March and April, the upper or 

 northern edge of this Light inclined more and more off from the ecliptic, and stretched on 

 farther to the northward; and knowing that the sun's equator, as shown by his spots, was also 

 now stretching off from the ecliptic in a similar way, he came to the conclusion that the sub- 

 stance giving this light was closely connected with the sun's equator, and was consequently 

 changing its position with that equator*. He argued further, that, as the sun has an atmo- 



CASSINI'S DIAGRAM. 



" Je fis reflexion que 1'e'quinoxial propre du soleil, qui est counu par le mouvement de ses taches qui se meuvent autour 

 de lui, dgcliaait alors [March and April] de l'e"cliptique selon apparence du cote" d'orient vers septentrion, et que cette de- 

 clinaison augmentait de Mars en Avril ; ce que me fit pen&er que le mouvement apparant de cette lumiere pourrait etre regie" 

 par celui du soleil autour de son axe, et la lumiere renvoye'e a peu pres selon le plan de son e'quinoxial. " Mfmoires de 

 I' Academic Royale dcs Sciences, tome viii,p. 127. 



