44 A. W. Wright—Spectrum of the Zodiacal Light. 
tent of the spectrum is not much lessened, and the different 
colors are still perfectly distinct. In the case previously men- 
tioned, where light from the sky was napa into a darkened 
room, and rendered extremely feeble, the result was different. 
The distinction of colors was nearly or quite impossible, and 
the extent of the spectrum much diminished. The red rays 
had become almost too faint to affect the eye sensibly, and the 
apparent point of greatest intensity had perceptibly moved up- 
ward, an effect, which is, of course, chiefly subjective. Its 
position was fou nd in various ways, as by moving the slides 
over the fuint spectrum till their edges were most distinctly 
seen, or by observing the highest point of the curve which is 
formed by the boundary of the spectrum, when produced with 
a wedge-shaped slit, or when a thin hollow wedge of glass filled 
with ink was placed before the slit. Repeated determinations 
of its place showed that it was between 70 and 80, but nearer 
the former, and in tlie plate it is placed at 74. This is perhaps 
a little too high, but it is the best approximation the difficulty 
of the determination allowed. Quite the same result was found 
with moonlight and twilight, and the maximum was sensibly 
at the same point as with light from the sky. In some even- 
ings during the early portion of the investigation, observations 
ect to the second point of the inquiry, evidence : 
was obtained which leaves little doubt that the bright line 
which has been occasionally seen, does not belong to the spec- 
trum of the zodiacal light. If the negative evidence of scores 
of observations in whiek it could not be detected is insufficient, 
there are facts which satisfactorily explain its presence on the 
few oceasions when it has appeared. Both Prof. Smyth * and 
M. Liais,+ who have made careful observations, deny that it 
belongs to the zodiacal s — and the former gives a con- 
clusive as platen of its supposed existence there. It is 
cient to recall the fact that the light which gives the 
uroral line is essentially monochromatic, and would be visible 
in the spectroscope even when with the naked eye it could not 
be detected in the general illumination of the sky, as it would 
not be weakened by dispersion like the latter, and would hence 
be relatively intensified. In the course of the many observa- 
tions made by the writer, the line was visible on three even- 
ings, but on each of these - occasions there was an aurora, which 
on one of the evenings had a considerable intensity. The 
* Monthly Notices of Royal Ast. Soc., loc. cit. 
+ Comptes Rendus, Tom. 74, 1872, p. 262. 
a 
* ag 
tee fe eee ela a i Ee ae 
