F. A. P. Barnard on the Zodiacal Light. 229 
planes begins, as before, at the setting of the sun, and continues 
until the preceding limb of the limiting circle touches the limb 
of the horizontal circle opposite to its first point of contact. Dur- 
ing this period, one cusp may possibly be visible, and in a right 
sphere probably would be. But after this second contact, neither 
could be seen until after midnight, the following limb of the lim- 
iting circle shouid touch the horizon at the original point of 
contact, when the second cusp might in its turn become visible, 
In this case, it is not matter of necessity that the light should be 
seen at all during the sun’s absence; nor that, when seen, it 
should extend over an enormous are of the heavens; and in these 
two particulars it best corresponds to the facts of ordinary ob- 
on. 
Servati 
for the ring from the earth’s centre which the observations may 
seem to warrant, we may ascertain what circumstances, if any, 
will limit the visibility of the light, what ought at any time, to 
: murposes at prese 
view, as if no such parallax existed. - But if, on the other hand, 
We find ourselves compelled (as, from what has already been said, 
enormous ; and the ring, though really lying in the plane of 
= ecliptic, can never nppene to be so, except when vertical. 
ate distance to which we have seen that the ordinary observa- 
ons would lead us—that is to say, a distance not exceeding two 
