224 F. A. P. Barnard on the Zodiacal Light. 
Mr. Jones himself furnishes an argument almost conclusive in 
disproof of the hypothesis of an illumination merely local, in his 
record of the aspect of the light as it appeared, on one or two oc- 
casions, in presence of the moon. He considers it to be well es- 
tablished that the ring was made visible by a reflection of the 
boundaries of the light. Yet if the sun produces a Juminous 
locus, there is no reason why the moon should not do the same ; 
and it is obvious that the two Joci, under the circumstances just 
described, could not possibly be coincident. On the other hand, 
the fact that the moon illuminated the ring in a line not passing 
through, but passing by itself, would seem to demonstrate pretty 
conclusively that the nearer margin of the brightness was the ac-» 
tual boundary of the ring. The circumstances must have been 
peculiarly favorable which brought the moveable reflection of 
the sun’s light at the same time, upon the same margin also. 
Upon this point there remains to be added but a single further 
observation. If the zodiacal light is but a luminous doeus, shift- 
ing its place as the observer moves, and occupying but a limited 
portion of a broad nebulous ring, then either this light should be 
more or less visible at all seasons and in all latitudes when the 
sun is below but near the horizon (and more conspicuously so in 
proportion as the ring is more distant from the earth) or at certain 
inclinations of the ecliptic to the vertical, it should grow narrower 
by losing a portion of its breadth on one side only, and should 
thus disappear by a gradual extinction proceeding from side to 
side. The first of these cases corresponds to that of a ring hav- 
ing a breadth at least equal to the earth’s diameter ; the secon 
supposes the breadth to be less than this diameter; and the 
inclination of the ecliptic which should cause the axis of the 
brightness to pass off from the ring may be found by the 
eq 
4 breadth of ring 
earth’s radius — 
sin. zenith dist. of ecliptic = 
These propositions which, if no lateral parallax is admitted, 
would represent rigidly the truth in regard toa nebulous ring con- 
centric with the earth, are believed not either of them to hold © 
the zodiacal light. — _ ; 
The third source of difficulty in regard to the ring theory !S 
found in the inconsistency of the results legitimately deduced 
from different observations. Assuming the theory to be true, the 
place of the summit of the light when visible must be deter- 
mined intersection of the uppermost strata of the ring with 
ng 
i A 
