440 H.C. Lewis—Zodiacal Light. 
altitude with a definite portion of the Via Lactea; and it was 
then always noticed that the diminution in brightness of the 
zodiacal cone was accompanied by a corresponding change in 
the Via Lactea. Any change in the transparency of the atmos- 
phere or in personal judgment affects equally both phenomena. 
Not only have no pulsations been observed, but as yet no 
periodic variations in the appearance or brightness of the zodi- 
acal cone have been noticed. e photometric observations 
mn enomenon generally ignored is thus brought to special notice. 
he observations of the writer tend to prove the invariability 
of the zodiacal light. The difference in its appearance 1s 
se to be due merely to the different positions of the earth 
in reference to it. 
Nor has the moon been discovered to have any action upon 
the appearance of the zodiacal cone. The zodiacal cone is fre- 
quently sufficiently bright to enable it to be seen when the 
moon is either in it or higher in the heavens. The presence 
of the moon does not appear to alter its shape. hen the 
moon is above the zodiacal cone, but not on the ecliptic, it has 
been frequently observed that the axis of the cone points away 
from the moon, making with it a considerable angle. Again, 
the cone is found to preserve its shape, both while the moon 
lies within it, or on one side of it, or when, after having passed 
its first quarter, the moon illuminates it from above. The 
widening of the base of the zodiacal cone, as the moon lights 
up the horizon, is an atmospheric effect caused by the bright- 
ening of the “ horizon light.” 
The writer has taken several observations upon the spectrum 
of the zodiacal cone. Three different spectroscopes, of different 
make,* have been used with accordant results. It was of interest 
to find that, notwithstanding the brilliancy of the cone when 
observed, nothing whatever could be seen when using a narrow 
slit,—a fact proving the truly continuous character of the spec- 
trum. When a slit of overa millimeter in width was used, 
there appeared a faint, pale, continuous spectrum, brightest and 
most abruptly ending at the less refrangible end, and gradually 
* Browning’s one prism, Browning’s direct vision, Eaton’s direct vision,—the 
last being the most satisfactory. 
