156 Professor Denison Olmsted on the 



Mairan in his celebrated Treatise on the Aurora Borealis, 

 including also a few observations of his own, and of several 

 other philosophers. It is interesting to compare these 

 ancient observations with such as we have been able to make 

 at corresponding times of the year; and having made this 

 comparison in numerous instances, I feel able to say, that the 

 zodiacal light, in the main, is the same thing that it was in 

 the days of Cassini and Mairati, being subject to similar 

 variations at different seasons of the year and in different 

 states of the atmosphere. I shall avail myself of such aid 

 as I can obtain from this and every other source in the re- 

 maining parts of this essay. 



Nature and Constitution of the Zodiacal Light. 



1. Length. — The extreme portions of this body sometimes 

 extend beyond the earth's orbit. It is obvious that, at an 

 elongation of 90°, it must reach a tangent drawn to the 

 earth's orbit at the place of the spectator; and if it reaches 

 beyond that tangent, as is sometimes the case, it must of 

 course extend beyond the earth's path. According to one of 

 our observations, on the 18th December 1837, its elongation 

 was 120°. The variable apparent elongation to which this 

 phenomenon is subject, is more or less influenced by three 

 causes ; the state of the atmosphere, the inclination of its 

 line of direction to the horizon, and the length of the twi- 

 light. In order to eliminate the effect due to atmospheric 

 changes we require numerous series of observations, con- 

 tinued through successive years, and, if possible, instituted 

 at long intervals of time. The mean of such an assem- 

 blage of observations would exhibit results nearly free from 

 the effects of accidental variations in the transparency of 

 the atmosphere. Since the axis of the zodiacal light does 

 not deviate far from the ecliptic, we may imagine it to 

 be represented by a portion of that circle on the artificial 

 globe, and we shall easily see that since its inclination to the 

 horizon varies between 25° and 72°, being 25° at the vernal 

 equinox (25° with the eastern, and 72° with the western 

 horizon), this cause mustgreatly affect the degree of intensity 

 of the zodiacal light. The same must obviously be the case 



