*314 PtoJ. Olmsted on the Zodiacal Light. 



March 29. — Light more faint. Elongation 60^ Vertex near the 



ecliptic. 



April 6. — Light fading rapidly. Very difTiise. 



May L — Last night a very plentiful rain after a series of warm 

 days. To-day air keen and sky very clear. This evening zodiacal 

 light remarkably distinct, (for this season of the year,) being discerna- 

 hle much nearer the horizon than common, and reaching further east- 

 ward among the stars than I ever observed it before, namely, into the 

 neighborhood of Castor and Pollux. Elongation 60'', but presumed to 

 be much greater than it would be but for the extraordinary transparency 

 of the atmosphere. 



May 10, 1834.— Zodiacal light seen for ten minutes after twilight 

 ceased — say till ten minutes after nine. Reached to Castor, but very 

 diffuse. Elongation 57^. 



Seen no more in the west till the latter part of November. 



To present at one view the various elongations from the sun, 

 observed from Nov. 2 1st to May 



1. Nov, 21st, Elongation, 90° 



2. Nov. 26th, '' 100° 



3. Dec. 2d, " 110° 



7. Feb. 7(h, Elongation, lo 



8. March 29th, " 60' 



9. April 6th, Light rapidly fading 

 4, Dec. 18ih, " 120° | 10. May 1st, Elongation, 60' 



5, Dec. 21st, " 90° 



6. Dec. 28th, " 75 



11. MaylOth, " 5T 



From this tabular view it appears that when the body first 

 came into view, on the 21st of November, it extended about 90^ 

 eastward of the sun; that its elongation increased rapidly from 

 this period, being five days afterwards 100^, in six days more 

 110°, and in fourteen days after this 120^, which is the greatest 

 elongation I have ever noticed; and being at the same time 

 about 60° westward of the sun, its whole extent in longitude 

 was 180°. 



I have, in a few instances, remarked what was apparently a 

 sudden and remarkable expansion of the zodiacal light, a circum- 

 stance more than once noted by Cassini. My record for Novem- 

 ber 21st, 1838; is as follows: 



At 5 A, M., about 20 minutes before twilight, the zodiacal light was 

 very large, extending in breadth from Corvus to Arcturus. Never saw 

 it so broad before. More inclined towards the south than usual, its 

 vertex passing one or two degrees to the south of Regulus. 



Whether this extraordinary enlargement in breadth, implying 

 a space of more than 40° was owing to a change in the body 

 itself, or to some unusual atmospheric refraction, or the acciden- 

 tal presence of an aurora borealiSj it is impossible for me to decide. 



It is well known that the great French astronomer, Dominique 

 Cassini, was the first to direct the attention of astronomers to- 

 wards the zodiacal light, and that he made numerous observa- 

 tions on it extending from 1683 to 1688 inclusive; which are pub- 



I 



