226 F. A. P. Barnard on the Zodiacal Light. 
The process is however greatly simpli- 3. 
fied, if we make the observation from a 
§ 
rently in the axis of the light and @ the 
summit. PO is the tangent (to the earth’s 
radius) of half the sun’s depression, and is 
therefore known. In the plane triangle 
OPQ, the angle at P is equal to the entire 
depression of the sun, and POQ is deter- . 
mined by observation. These, with the known base, OP, will 
give OQ, from which the height of the ring above the earth’s 
surface may be determined as before. 
ow to make an application of this method, not with a view@ 
to obtain a result perfectly accurate, for which no observations 
sufficiently precise are at hand, but in order to find an outside 
limit to the possible altitude of the visible ring; it is safe to as- 
sume that the luminosity has never been seen extending more 
from most of the observations hitherto made of this phenomenon, 
whether by Mr. Jones or by any other observer. The probabil- 
ity is that the results of such an examination would all be more 
or less discordant with each other; while it is manifest that, 1 @ 
few instances, we should reach conclusions singularly at variance 
with the mass. Mr. Jones several times observed the light simul- 
taneously in the east and west horizons, when the sun was de- 
h 
ude—as in fact his diagrams 
height, if it was stated by him, 
