404 ANNUAL OF SCIENTIFIC DISCOVERY, 



tic, usually a little to the north or south of it, but sometimes corresponding 



with it. 



The facts observed by him from observation, condensed, are as follows : 



1 . The zodiacal light occupies a constant relative position in the plane of 

 the ecliptic, preceding or following the sun. 



2. Its central line is pai-allel with and coincides with the ecliptic. 



3. Its apex varies in distance sixty to one hundred and ten degrees from 

 the sun. Its height above the horizon seldom exceeds sixty degrees. 



4. Its azimuth changes with the sun, and with the observer's position on 

 the earth. 



5. Its inclination alters with the position of the observer in latitude, from 

 the vertical down to an acute angle with the horizon. 



6. The morning and evening zodiacal light are different in phase, color, 

 altitude, and inclination, depending upon the angle subtended between the 

 observer's horizon and the plane of the ecliptic. 



7. Its apex lies always south of the zenith when the observer is north of 

 the ecliptic, and north of the zenith when he is to the south of the ecliptic. 



8. When the ecliptic passes through the zenith of the observer, the column 

 of light is vertical to the horizon ; it then assumes the appearance of a nar- 

 row belt, with a well defined apex. 



9. North or south of the ecliptic, the zodiacal light exhibits a broader 

 phase, but less in altitude than when under it. 



10. The zodiacal light is never seen until the sun has set and twilight 

 ended, or until all reflected light is cut off; therefore, visibility depends upon 

 the continuance of twilight. 



11. Owing to the length of twilight, the zodiacal light is seldom ever near 

 the limiting parallel. The limiting parallels vary with the sun's declination. 



12. The sun's rays falling perpendicularly on the atmosphere within the 

 tropics, are not reflected ; consequently after sunset there is little or no twi- 

 light. 



These facts go to prove that this phenomenon is the result of the illumin- 

 ation of that portion or section of the earth's atmosphere on which the rays 

 of the sun fall perpendicularly. It will readily be conceived that rays of 

 light will illuminate a column or portion of atmosphere on which they may 

 fall, when no reflected or diffusive light interferes to prevent its being visible. 

 To illustrate : if the direct rays of the sun are admitted through a hole in a 

 shutter into a darkened room, the atmosphere and the particles floating in it 

 become as visible and distinct when all reflected light is cut off as any well 

 defined object. It is this which we believe takes place when the rays of the 

 sun fall perpendicularly on our atmosphere, and produce such an effect which 

 becomes visible upon the earth's surface when this column is above the hori- 

 zon, Avithin the limiting parallels, and after the twilight lias ceased. The 

 whole earth is constantly exposed to and revolves in the sun's rays. A part 

 of these rays only fall perpendicularly on our atmosphere, while all others 

 strike it obliquely, are reflected and refracted by it. As the earth revolves, 

 this column or section of the atmosphere which lies in the ecliptic or earth's 

 orbit becomes illuminated in succession, and thus appears permanently at- 



