1889 — 90 .] Prof. C. Michie Smith on the Zodiacal Light. 145 
above mean sea-level. The notes made on that occasion include the 
remark “ certainly no bright lines.” 
It appears, then, that in all my observations, which have been 
carried on at intervals since 1875, the spectrum has appeared con- 
tinuous and free from bright lines except during the spring of 1883, 
and that even then the lines were not seen with sufficient distinct- 
ness to make their existence certain. The estimated position of the 
supposed line, W. L. 558, differs but little from that of the auroral 
line (W. L. 556’7) which was observed by Angstrom* in the 
zodiacal light spectrum in 1867. He, however, was observing at 
Upsala, where the auroral spectrum can often be seen in almost all 
parts of the sky, even when the aurora itself cannot be detected. 
Senor A. T. Acrimis, f observing at Cadiz, saw two bright lines in 
the spectrum, but since he used a refractor and a five prism spectro- 
scope it is almost certain that it was not the spectrum of the 
zodiacal light that he saw. There would seem to be very little risk 
of obtaining the auroral spectrum in Madras, and I think that if the 
bright line seen was real and not imaginary it must have been due 
to the zodiacal light. It seems, too, to be in favour of its reality 
that my own prepossessions — arising from hundreds of previous 
observations, were entirely against the existence of such a line. 
These observations are presented to the Society with great hesita- 
tion, but as they were made with every precaution that I could 
devise, I think it best to place them on record. 
Another point with reference to the zodiacal light on which more 
detailed and accurate observations are much needed is its exact 
position in the sky. It was only recently that I discovered how 
very unsatisfactory existing determinations were, and I do not 
propose at present to give any results of my own measurements. I 
wish, however, to call attention to one or two points. Very little 
weight can be laid on determinations of the inclination of the axis 
of the light made in high latitudes, for Searle has shownj that the 
difference in absorption between the upper and lower boundaries may 
produce a very considerable displacement of the apparent axis. 
But no such explanation will account for the differences shown in 
* Pogg. Annul., vol. 137, p. 162. 
t R. A. S. Monthly Notices, xxxvi. p. 1. 
+ Proc. Amer. Acad., vol. xix. p. 146 ; Memoirs Amer. Acad., xi. p. 135. 
VOL. XVII. 26/5/90 K 
