310 PtoJ, Olmsted on the Zodiacal Light. 



number of years are necessary of diligent attention to the phe- 

 nomena of this light, in order to become well acquainted with its 

 habitudes and laws. Nor can I pretend to have made the best 

 possible use of the opportunities afforded for viewing it, during 

 the six years that my attention was directed to it. On the con- 

 trary, my observations were often interrupted by ill heahh, and 

 other causes beyond my control. Still, they were sufficient to 

 convince me that my previous knowledge of this body was ex- 

 ceedingly defective, and my notions of it very erroneous ; and 

 the same may justly be said of most or all of the descriptions and 



graphic representations of it given in works of science. 



I will therefore; first, attempt an accurate description and rep- 

 resentatio7i of the zodiacal light. 



Since the direction of this body is oblique to the circles of di- 

 urnal revolution, and since it appears only immediately before or 

 immediately after the sun, and therefore more or less of it falls 

 within the twilight, consequently, its appearances are very differ- 

 ent in different latitudes, being seen best of all in the tropical re- 

 gions, where its direction always makes a high angle with the 

 horizon, and where the twilight is short ; and being scarcely visi- 



ble in such high latitudes as London and Edinburgh, except near 

 the time of the equinoxes. Hence British writers who have at- 

 tempted a description of it, have usually given one that is alto- 

 gether vague and inaccurate. The lower latitude of our place of 

 observation (41^ IS' 30'^) affords a much better view of it, and 

 my description and representation of it will conform to its appear- 

 ance at this latitude. 



I learn from my friend, Prof. Dana, that w^hile with the Explo- 

 ring Expedition in the torrid zone, he seldom failed of seeing the 

 zodiacal light morning or evening, when not prevented 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 au- 

 tumn we look for it in the morning sky, and at the end of au- 

 tumn in the evening sky. The state'of the atmosphere most 



favorable for seeing it at its minimum 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 see more distinctly the faint 

 diffusive light of the upper portions. With these advantages we 

 may unite that of fixing one eye on a darker portion of the heav- 

 ens a few degrees to the right or left, and looking askance with 

 the other eye over the region of the object sought. This last 

 expedient 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 northeast early in 

 August, yet it will hardly be obvious to common observation be- 



