THE ZODIACAL LIGHT. ' 351 



other stellar bodies. Traversing space with inestimable velocity and 

 performing their revolutions with unvaried regularity, they have long 

 been known as part of our solar system. It seems sti'ange that a cos- 

 mical body so near the earth as the zodiacal light should have re- 

 ceived comparatively so little attention. 



As many who read this article have never seen this light, it is 

 necessary that it be described. 



It is defined, in the work under review, to be " a brightness that 

 appears in the western sky after sunset, and in the east before sun- 

 rise ; following nearly or quite the line of the ecliptic in the heavens, 

 and stretching upward to various elevations according to the season 

 of the year." Tliere is a slight objection to this definition, by which 

 inexperienced observers maybe led astray. It is spoken of as appear- 

 ing after sunset, by which some would be led to suppose that it is 

 visible immediately after sunset ; whereas it is never to be seen until 

 after the night has fully set in, and the sun's rays are some distance 

 below the horizon. Its varied elevation, indeed its appearance, is also 

 dependent upon the latitude of the observer as well as the season, so 

 much so, that in very high latitudes it is but seldom seen to advantage. 

 It has also been seen at almost every hour of the night, but is usually 

 more distinctly seen in the temperate zones, when observed between 

 dark and nine o'clock, as after that hour its light frequently becomes 

 dim and diffuse. 



" It appears to best advantage when the ecliptic makes its highest angle with 

 the spectator's horizon, at which time, in moderate latitudes, it reaches to the 

 zenith or beyond it, liaving near the horizon a striking brilliancy, and thence 

 fading upward, mostly by imperceptible degrees, till at its vertex it can be made 

 out only by a careful and experienced eye. As the seasons advance, when the 

 ecliptic is declining gradually toward the horizon, the zodiacal light fades away 

 till it is perhaps entirely lost to view, or can be seen only by those who have 

 followed it in its changes, night after night, and are thus able, by famihar ac- 

 quaintance, to detect and trace its dim markings in the sky." 



Humboldt, in " The Cosmos," vol, i., remarks : 



" Those who have been for many years in the zone of palms must retain a 

 pleasing impression of the mild radiance with which the zodiacal light, shoot- 

 ing pyramidically upward, illumines a part of the uniform length of tropical 

 nights. I have seen it sliine with an intensity of light equal to the milky- way 

 in Sagittarius, and that not only in the rare and dry atmosphere of the summit 

 of the Andes, at an elevation of from 13,000 to 15,000 feet, but even on the 

 boundless grassy plains, the llanos of Venezuela, and on the sea-shore beneath 

 the ever-clear sky of Cumana." 



Prof. Olmsted, in his " Astronomy," describes it as follows : 



" Its form is that of a luminous pyramid, having its base toward the sun. It 

 reaches to an immense distance from the sun, sometimes even beyond the orbit 

 of the earth. It is brighter in the parts nearest the sun than in those that are 

 more remote, and terminates in an obtuse apex, its light fading away by insen- 



