150 Professor Denison Olmsted on the 



aftbrd a much better view of it, and my description and 

 representation of it will conform to its appearance at this 

 latitude. 



I learn from my friend Professor Dana, that while with the 

 exploring expedition in the torrid zone, he seldom failed of 

 seeing the zodiacal light morning or evening, when not pre- 

 vented by some of the causes before enumerated ; but during 

 the summer months in our climate we hardly see it at all. 

 At the beginning of autumn we look for it in the morning sky, 

 and at the end of autumn, in the evening sky. The state of 

 the atmosphere most favourable for seeing it at its mini- 

 mum intensity, is that peculiarly transparent condition which 

 either precedes or follows a copious rain. The presence of a 

 black cloud, also, near the horizon, frequently enables us, by 

 contrast, to pee more distinctly the faint diifusive light of the 

 upper portions. With these advantages we may unite that 

 of fixing one eye on a darker portion of the heavens a few 

 degrees to the right or left, and looking askance with the 

 other eye over the region of the object sought ; this last expe- 

 dient will usually be found useful for fixing its exact boun- 

 daries, in its various stages of intensity. 



Although, as was first remarked by Mr E. C. Herrick, faint 

 traces of the zodiacal light may be seen in the north-east 

 early in August, yet it will hardly be obvious to common 

 observation before the latter part of September. I quote 

 from my record for September 25th, 1835 : — 



" Observed the zodiacal light from 3 to 4^ o'clock A.M. Very 

 faint. Seen only by fixing the right eye on the region of 

 Canis Major, and carrying the left eye along the ecliptic. 

 Covers Regulus and the cluster in Cancer, and terminates 

 a little south of Castor." 



The earliest distinct view I have obtained of this body in 

 the evening sky, was on the 21st November 1837, when I 

 have the following record : — 



" Have constantly searched for the zodiacal light in the 

 evening since the 13th inst. I imagined that that part of the 

 milky way where this light would cross it was more luminous 

 than common, but the light is ambiguous on account of the 

 presence of Venus. But this evening examined in company 



