F. A. P, Barnard on the Zodiacal Light. 223 
the brightest (so far as there is a difference) will be that which is 
in the vertical passing through the sun ; and, therefore, that, if the 
nebula is extensive enough, this same vertical plane, by its inter- 
section with the nebula, will determine the docus of principal 
brightness. It can rarely happen, however, that the ecliptic will 
pass through the’ observer’s zenith; and when it happens, the 
state of things can be but momentary. The fact therefore, that 
the axis of the zodiacal light appears to be always in the ecliptic, 
and that it is not displaced laterally by any change of position 
in the observer, appears to the writer to furnish a proof almost 
decisive that this light cannot proceed from a nebulous ring en- 
circling the earth. 
_ In admitting, in the foregoing argument, the possibility that the 
light reflected to a nebulous body may have an axis of maximum 
#@pparent brightness, it will be observed that the admission rests 
upon the supposition that, in a certain plane, the angle between 
imum. This, ho 
horizon when the sun is ninety degrees below, it is impossible 
that its brightness should be ‘restricted to a limited portion of its 
breadth. Yet the delineations of the zodiacal light, as observed 
under these circumstances, in Mr. Jones’s diagrams, presented no 
peculiarity—certainly no enlargement of breadth. We can form 
ho other conclusion, therefore, but that the nebulous body from 
which this light proceeds, is always seen illuminated throughout 
its Whole extent ; and consequently that the absence of a paral- 
lax is inadequately accounted for by the explanation suggested 
by Mr. Jones. 
The 
an illuminated nebula may be expected to present, are confirmed 
by the actual phenomena of twilight. When the sun is very neat 
to more intense reflection, but in a measure to the greater depth 
(in the direction of vision) of the mass of air illuminated. Ata 
ce tr | 
ers of the western horizon, we observe no greater inequality of 
brightness throughout the whole extent of the illumination, 
tee 1s fairly attributable to the varying depth of the mass 
