F. A. P. Barnard on the Zodiacal Light. 221 
remote from the earth, and seen by reflection just previously to 
the morning, or at the close of the evening twilight, ought to 
present some characteristics which are wanting in the zodiacal 
light. That it is never visible in the quarter of the heavens op- 
posite to the sun, may be attributed to the fact that it there falls 
into the shadow of the earth; but toward the sun its form should 
be unlike what it is. Considering it as limited by the shadow, 
we should expect that its upper portion should display a bright- 
ness gradually fading, (as it actually does,) but we certainly 
10uld not look to see this fading commence from the very hori- 
zon itself. If, ig the annexed fig- 1. 
ure, EK be the earth, SW the tan- 
gent limiting the shadow, HR, the 
orizon, and PP’Q’Q the portion 
of a nebulous ring concentric with 
the earth visible to the observer at 
Said, on the other hand, that the horizontal ray is much more 
the ring at Q’ is not seriously affected by the obj 
we know that, at the very earliest moment at which we are able 
'o observe this light after sunset, (which is the elose of twilight, ) 
the appearance presented is that of an uninterrupted diminution 
both of breadth and brightness, from the very horizon Z 1. 
ere is certainly no limit, as at Q’, dividing a part, QP’, uni- 
formly bright, or decreasing downward in brightness, fro S 
