42 A. W. Wright—Spectrum of the Zodiacal Light. 
duce a definite and constant visual impression. They may serve 
for comparison with those of other spectra measured in the same 
way. It could plainly be seen, on slowly moving the slides 
toward the end of the spectrum, and just before it ceased to be 
visible, that the light extended a considerable distance beyond 
them, as much perhaps as ten or fifteen divisions of the scale. 
The position of maximum brightness was merely estimated, 
and was not perceptibly different from that of the twilight spec- 
trum. The extent and general form of the spectrum are shown 
in the accompanying plate (Plate 11), where it is marked L 
The appearance of the spectrum, as seen when the slit had a 
breadth of only two scale-divisions, is represented in No. V, 
but the limits there indicated are less certain than those ob- 
tained in the other cases. 
number of measurements were made, in the same manner, 
t 
gradual variation of intensity, but was between 70 and 
80, apparently somewhat nearer the former. It is placed at 
74 on the plate, spectrum IV. 
Observations on light from the moon, and upon twilight, 
showed that their spectra correspond even more closely with 
that of the zodiacal hght. In the case of the former, the lunar 
rays were received upon a white unglazed card placed a few 
inches in front of the slit. The latter being narrowed to two 
scale-divisions, the limits were found to be 47 and 136-6, the 
