OF I-IEAT FEOM THE MOON. 
621 
Four determinations made by Professor Zollnee after his diagram was engraved are 
here given, and two errors pointed out by him* in the elongations of two of Sir John 
Heesciiel’s observations (caused by errors in the ‘Cape Observations’) have been rectified. 
The dotted line is Professor Zollnee’s calculated curve, the ordinates of which have 
been increased in the ratio of 4*880 to 1 3 so as to make it agree as closely as possible 
with the heat-curve f. 
On inspecting the diagram, it is at once apparent that the increase of the moon’s light 
hi approaching the full moon is more rapid than that of her heat, so much so that 
Zollnee resorted to a cusped curve for its representation. The introduction of the 
additional observations, however, three of which are upper ones in elongations —27°, 
— 24°, and — 11°, would not only seem to indicate a necessity for rounding off the maxi- 
mum of the light-curve, but also for placing it slightly before full moon, and thus making 
it agree with the heat-curve in this remarkable feature. 
Some uncertainty appears to arise on account of the employment by Zollnee of a 
photometer in which, when used on the moon, the light has to traverse a system of 
Nicol prisms ; but it does not appear clear in what azimuth the system was turned 
during each observation, and consequently it is not known whether the correction due 
to this cause would diminish the departure of the heat- from the light-curve, or the 
reverse J. 
Attempt to compare the Moons Radiant Heat with that from a terrestrial source. 
The effect of the moon’s heat has hitherto been expressed in this paper on a purely 
arbitrary scale, namely by the differences of the readings of one and the same galva- 
* Photometrische Untersuchungen, p. 175, note. 
t In accordance with the heading of the Table (Phot. Unt. p. 198) the curve is kept within the limits of 50° 
before and 70° after full moon. 
+ Taking the extreme ease, where the plane in which the sun, moon, and earth, and therefore the plane in 
which the moon’s light is polarized, .is parallel to the plane of reflection of the transparent plate of parallel glass 
in the photometer, the system of Nicols being supposed to be set with the principal axis parallel to this plane, 
the phase-curve obtained would differ from the heat-curve by only about two thirds the present amount. Were 
the system of Hicols moved round through 90°, the correction to be applied to the curve would be in the other 
direction. 
It has been assumed that the' ratio of the two components of natural light after passing through the plate of 
glass is 0-84 to 1-00 at 45° incidence, and the corresponding ratio for moonlight 0-83 to 1-00 (probably 
too high an estimate of the mean polarization of the moon’s light), at quadrature the maximum polarization 
occurring at aborrt 77° elongation. The ratio of intensities found in the two cases would thus be the same at 
about 85° from full moon, and their ratio 084 to 1-00 at full moon. Sir J. Hekschel employed a photometer 
which appears to be free from this source of error. 
I cannot find that any one has devoted much attention to the subject of the polarization of light from the 
moon except Arago and Father Secchi. The former states that the maximum polarization occurs at or near 
quadrature, but gives no estimate of its amount. Our experiments in this direction are not as accordant as 
might be wished, and for the present do not appear worth publishing. 
It may, however, be well that any who happen to be working at Photometry should have their attention in 
the mean time called to this possible source of error. 
4 o 2 
