PRELIMINARY NOTE ON SOME MEASUREMENTS OF THE 
POLARIZATION OF THE LIGHT COMING FROM THE 
MOON AND FROM THE PLANET VENUS. 
BY 
THE EARL OF ROSSE, Fras. 
[Read and handed in, Monday, May 21, 1877.] 
SEVERAL years ago, at the suggestion of a friend, having examined 
some portions of the lunar surface with a Nicol’s prism with a 
view to the detection of small sheets of standing water, if any 
such chanced to.exist, I was led on to make a rather extended 
examination of particular portions of the surface with the pola- 
rimeter, under the idea that if the precise position of elongation 
from the sun where the polarization of a point of the lunar surface 
attains a maximum could be accurately determined, it might be 
possible to obtain an approximate value of the refractive index of 
the material composing that surface, and so to distinguish between 
material of a vitreous nature, ejected from volcanoes, and a surface 
of ice and snow. 
The subject has been invested with the greater interest from 
the fact that Arago, having found the maximum of polarization 
of the whole of the moon’s light to occur at, or near quadrature, 
remarks on the circumstance as being what might be expected to 
result from the reflecting surface being gaseous; and he appears 
to think that the polarimeter may afford us some information on 
the question of the existence of a lunar atmosphere.* 
During the years 1872—3-4-5 we have at intervals made a 
rather extended series of measurements with the polarimeter, of 
which several different forms were tried, but one differing in 
little from Arago’s, was found the more satisfactory. 
Although from a series of sixteen readings of the inclination of 
the plates of parallel glass, a value may be obtained for the 
polarization on each night with a probable error of observation 
not exceeding one per cent., from some cause not yet established, 
the discrepancies between the various nights’ work are much 
larger, and the results must be accepted with reserve and con- 
sidered as only provisional. 
* Arago: Cuvres, nouyv. ed. par Barral, liv. xiv., chap. vi., t. ii., p. LOL, &e. 
VOL. I—PART. L : C2 
